PURCHASE AGREEMENT

Exhibit 10.29

ACAS BUSINESS LOAN TRUST 2005-1

ASSET-BACKED NOTES

SERIES 2005-1

$435,000,000 CLASS A-1 NOTES

$50,000,000 CLASS A-2B NOTES

$50,000,000 CLASS B NOTES

$145,000,000 CLASS C NOTES

PURCHASE AGREEMENT

September 29, 2005

Wachovia Capital Markets, LLC

Citigroup Global Markets Inc.

Banc of America Securities LLC

BB&T Capital Markets,

    a division of Scott & Stringfellow, Inc.

Harris Nesbitt Corp.

HVB Capital Markets, Inc.

as the Initial Purchasers (the “ Initial Purchasers ”)

c/o Wachovia Capital Markets, LLC

as representative of the Initial Purchasers (the “ Representative ”)

One Wachovia Center

301 South College Street

Charlotte, North Carolina 28288

Ladies and Gentlemen:

American Capital Strategies, Ltd. (“ American Capital ” or the “ Company ”), ACAS Business Loan LLC, 2005-1 (the “ Trust Depositor ”) and ACAS Business Loan Trust 2005-1, a Delaware statutory trust (the “ Trust ”) have duly authorized the sale of the ACAS Business Loan Trust Notes, Series 2005-1, consisting of the Class A-1 Floating Rate Asset Backed Notes (the “ Class A-1 Notes ”), the Class A-2A Delayed Draw Floating Rate Asset Backed Notes (the “ Class A-2A Notes ”), the Class A-2B Floating Rate Asset Backed Notes (the “ Class A-2B Notes ” and, together with the Class A-1 Notes and the Class A-2A Notes, the “ Class A Notes ”), the Class B Floating Rate Deferrable Asset Backed Notes (the “ Class B Notes ”), the Class C Floating Rate Deferrable Asset Backed Notes (the “ Class C Notes ”, together with the Class A Notes and the Class B Notes the “ Offered Notes ”), the Class D Principal Only Asset Backed Notes (the “ Class D Notes ”) and the Class E Principal Only Asset Backed Note (the “ Class E Note ” and together with the Offered Notes and the Class D Notes, the “ Notes ”) of the Trust. The Trust is governed by an Amended and Restated Trust Agreement, dated as of October 4, 2005 (the “ Trust Agreement ”), among the Trust Depositor, Wachovia Bank of Delaware, National

Association, as owner trustee (the “ Owner Trustee ”) and the Company, as servicer. The Class A-1 Notes will be issued in an aggregate initial principal amount of $435,000,000, the Class A-2A Notes will be issued in an aggregate initial principal amount of up to $150,000,000, the Class A-2B Notes will be issued in aggregate initial principal amount of $50,000,000, the Class B Notes will be issued in an aggregate initial principal amount of $50,000,000, the Class C Notes will be issued in an aggregate initial principal amount of $145,000,000, the Class D Notes will be issued in an aggregate initial principal amount of $90,000,000, and the Class E Note will be issued in an aggregate initial principal amount of $80,000,000. For each Interest Accrual Period, the Class A-1 Notes shall bear interest at a per annum rate equal to the then applicable LIBOR plus 0.25%  per annum , Class A-2A Notes shall bear interest at a per annum rate equal to the then applicable LIBOR plus 0.20%  per annum , Class A-2B Notes shall bear interest at a per annum rate equal to the then applicable LIBOR plus 0.35%  per annum , the Class B Notes shall bear interest at a per annum rate equal to the then applicable LIBOR plus 0.40%  per annum and the Class C Notes will bear interest at a per annum rate equal to the then applicable LIBOR plus 0.85%  per annum ; provided , however , that the LIBOR rate on the Offered Notes for the initial Interest Accrual Period is a four-month LIBOR rate and for each Interest Accrual Period thereafter will be Three-Month LIBOR. The Notes will be issued pursuant to an Indenture, dated as of October 4, 2005 (the “ Indenture ”), between the Trust and Wells Fargo Bank, National Association, as Indenture Trustee (the “ Indenture Trustee ”). The Notes will be secured by the assets of the Trust.

In addition to the Notes, the Trust is issuing a certificate (the “ Certificate ”). The Certificate will represent fractional undivided ownership interests in the Trust. The Certificate will be issued pursuant to the Trust Agreement.

The primary assets of the Trust will be a pool of commercial business loans, or interests therein, originated or acquired by the Company from time to time (collectively, the “ Business Loans ”). The Trust Depositor will acquire the Business Loans and certain related assets from the Company pursuant to the ACAS Transfer Agreement, dated as of October 4, 2005 (the “ Transfer Agreement ”) between the Company and the Trust Depositor. Pursuant to a Transfer and Servicing Agreement, dated as of October 4, 2005 (the “ Transfer and Servicing Agreement ”), among the Trust, the Company, the Trust Depositor and the Indenture Trustee, the Trust Depositor will sell, transfer and convey to the Trust, without recourse, all of its right, title and interest in the Business Loans and certain related assets to the Trust. Pursuant to the Indenture, as security for the indebtedness represented by the Notes, the Trust will pledge and grant to the Indenture Trustee a security interest in the Business Loans and certain related assets and its rights under the Transfer Agreement and the Transfer and Servicing Agreement. This Purchase Agreement (the “ Agreement ”), the Trust Agreement, the Transfer Agreement, the Transfer and Servicing Agreement and the Indenture are referred to collectively as the “ Transaction Documents .” Capitalized terms used but not defined herein shall have the meanings provided in the Transfer and Servicing Agreement.

The Offered Notes are to be offered without being registered under the Securities Act of 1933, as amended (the “ Securities Act ”), to “qualified institutional buyers” in compliance with the exemption from registration provided by Rule 144A under the Securities Act (“ QIBs ”) who are “qualified purchasers” (“ Qualified Purchasers ”) for purposes of Section 3(c)(7) of the Investment Company Act of 1940, as amended (the “ 1940 Act ”), in offshore transactions in

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reliance on Regulation S under the Securities Act (“ Regulation S ”) except with respect of the Class A-2A Notes prior to the Commitment Termination Date, to QIBs in compliance with Section 4(2) of the Securities Act who are Qualified Purchasers, and to institutional “accredited investors” (as defined in Rule 501(a)(l), (2), (3) or (7) under Regulation D of the Securities Act) (“ Institutional Accredited Investors ”) who are Qualified Purchasers that deliver a letter in the form of Exhibit D-1 to the Indenture.

In connection with the sale of the Offered Notes, the Company has prepared a preliminary offering memorandum dated September 14, 2005 (including any exhibits thereto, the “ Preliminary Memorandum ”) and a final confidential offering memorandum dated the date hereof (including any exhibits, amendments or supplements thereto, the “ Final Memorandum ”, and each of the Preliminary Memorandum and the Final Memorandum, a “ Memorandum ”) including a description of the terms of the Offered Notes, the terms of the offering, and a description of the Trust.

The Company, the Trust Depositor and the Trust hereby agree with you, as the Initial Purchasers, as follows:

(a) Subject to the terms and conditions and in reliance upon the representations and warranties set forth herein, the Trust agrees to sell to the Initial Purchasers the Class A-1 Notes, the Class A-2B Notes, the Class B Notes and the Class C Notes (collectively, the “ Purchased Notes ”), and the Representative on behalf of the several Initial Purchasers has agreed to purchase the aggregate principal amount of Purchased Notes set forth on Schedule I hereto at the purchase price set forth thereon. It is understood and agreed that the Initial Purchasers are not acquiring, and have no obligation to acquire, the Class A-2A Notes, the Class D Notes, the Class E Note or the Certificate.

(b) In addition, whether or not the transaction contemplated hereby shall be consummated, the Company agrees to pay all other costs and expenses incident to the performance by the Company of its obligations hereunder and under the documents to be executed and delivered in connection with the offering, issuance, sale and delivery of the Notes, including, without limitation or duplication: (i) the fees and disbursements of counsel to the Company; (ii) the fees and expenses of any trustees or custodian due to such trustees’ or custodian’s initial expenses incurred in connection with the issuance of the Notes and its counsel; (iii) the fees and expenses of any bank establishing and maintaining accounts on behalf of the holders of the Notes or in connection with the transaction; (iv) the fees and expenses of the accountants for the Company, including the fees for the “comfort letters” or “agreed-upon procedures letters” required by the Initial Purchasers, any rating agency or any purchaser in connection with the offering, sale, issuance and delivery of the Notes; (v) all expenses incurred in connection with the preparation and distribution of the Preliminary Memorandum and the Final Memorandum and other disclosure materials prepared and distributed and all expenses incurred in connection with the preparation and distribution of the Transaction Documents; (vi) the fees charged by any securities rating agency for rating the Offered Notes and the Class D Notes; (vii) the fees for any securities identification service for any CUSIP or similar identification number required by the purchasers or requested by the Initial Purchasers; (viii) the

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fees and disbursements of counsel to the Initial Purchasers; (ix) all expenses in connection with the qualification of the Offered Notes for offering and sale under state securities laws, including the reasonable fees and disbursements of counsel and, if requested by the Initial Purchasers, the cost of the preparation and reproduction of any “blue sky” or legal investment memoranda; (x) any federal, state or local taxes, registration or filing fees (including Uniform Commercial Code financing statements) or other similar payments to any federal, state or local governmental authority in connection with the offering, sale, issuance and delivery of the Offered Notes; and (xi) the reasonable fees and expenses of any special counsel or other experts required to be retained to provide advice, opinions or assistance in connection with the offering, issuance, sale and delivery of the Notes.

(c) It is understood and agreed that nothing in this Agreement shall prevent the Initial Purchasers from entering into any agency agreements, underwriting agreements or other similar agreements governing the offer and sale of securities with any issuer or issuers of securities, and nothing contained herein shall be construed in any way as precluding or restricting the Initial Purchasers’ right to sell or offer for sale any securities issued by any person, including securities similar to, or competing with, the Notes.

Delivery of the Offered Notes shall be made in the form of one or more global certificates delivered to the Indenture Trustee, as custodian for the nominee of The Depository Trust Company, except that any Purchased Note to be sold by the Initial Purchasers to an Institutional Accredited Investor that is not a QIB shall be delivered in fully registered, certificated form in the minimum denominations set forth in the Final Memorandum at the offices of Dechert LLP, Charlotte, North Carolina at 10:00 a.m. Charlotte, North Carolina time, on October 4, 2005, or such other place, time or date as may be mutually agreed upon by the Initial Purchasers and the Company (the “ Closing Date ”) against payment by the Initial Purchasers of the purchase price thereof to or upon the order of the Company (on behalf of the Trust) by wire transfer payable in same-day funds to the account specified by the Company. Subject to the foregoing, the Purchased Notes will be registered in such names and such denominations as the Initial Purchasers shall specify in writing to the Company and the Indenture Trustee prior to the Closing Date.

The Company represents and warrants to the Initial Purchasers, as of the Closing Date, that:

(a) Each Memorandum does not and will not, and any amendments thereof or supplement thereto and any additional information and documents concerning the Notes delivered by or on behalf of the Company to prospective purchasers of the Offered Notes (collectively, such additional information and documents, the “ Additional Offering Documents ”), each as of their respective dates or the date on which such statement was made and as of the Closing Date, do not and will not include an untrue statement of a material fact or omit to state a material fact necessary in order to make the statements in each, in light of the circumstances under which they were made, not misleading; provided , that, the Company makes

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no representation or warranty as to the information contained in or omitted from either Memorandum, or any amendment or supplement thereto, in reliance upon and in conformity with information furnished in writing to the Company by or on behalf of the Initial Purchasers specifically for inclusion therein.

(b) The Company is a Delaware corporation, duly organized and validly existing under the laws of the state of Delaware, has all power and authority necessary to own or hold its properties and conduct its business in which it is engaged as described in each Memorandum and has all licenses necessary to carry on its business as it is now being conducted and is licensed and qualified in each jurisdiction in which the conduct of its business (including without limitation the originating and acquiring of Business Loans and performing its obligations hereunder and under the Transaction Documents) requires such licensing or qualification.

(c) This Agreement has been duly authorized, executed and delivered by the Company, and, assuming due authorization, execution and delivery thereof by the other parties hereto, constitutes a valid and legally binding obligation of the Company enforceable against the Company in accordance with its terms, subject to the effect of bankruptcy, insolvency, reorganization, moratorium and other similar laws relating to or affecting creditors’ rights generally or the application of equitable principles in any proceeding, whether at law or in equity.

(d) The Transfer Agreement and the Transfer and Servicing Agreement have been duly authorized, executed and delivered by the Company and, assuming due authorization, execution and delivery thereof by the other parties thereto, constitute valid and binding obligations of the Company, enforceable against the Company in accordance with their respective terms, subject to the effect of bankruptcy, insolvency, reorganization, moratorium and other similar laws relating to or affecting creditors’ rights generally or the application of equitable principles in any proceeding, whether at law or in equity.

(e) The Offered Notes have been duly authorized, and when executed and authenticated in accordance with the Indenture and delivered to and paid for by the Initial Purchasers in accordance with this Agreement, the Offered Notes will constitute valid and binding obligations of the Trust, enforceable against the Trust in accordance with their terms, subject to the effect of bankruptcy, insolvency, reorganization, moratorium and other similar laws relating to or affecting creditors’ rights generally or the application of equitable principles in any proceeding, whether at law or in equity, and will be entitled to the benefits of the Indenture.

(f) Other than as set forth in or contemplated by each Memorandum, there are no legal or governmental proceedings pending to which the Company is a party or of which any property or assets of the Company are the subject of which, if determined adversely to the Company, would individually or in the aggregate have a material adverse effect on the financial position, business or results of operations of the Company and its subsidiaries taken as a whole or on the performance by the Company of its obligations hereunder or under the Transaction Documents (a “ Material Adverse Effect ”); and, to the best knowledge of the Company, no such proceedings are threatened or contemplated by governmental authorities or threatened by others.

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(g) The execution, delivery and performance of this Agreement and the other Transaction Documents to which it is a party by the Company and the consummation by the Company of the transactions contemplated herein and therein and in all documents relating to the Notes will not result in any material breach or violation of, or constitute a material default under, any agreement or instrument to which the Company is a party or to which any of its properties or assets are subject, except for such of the foregoing as to which relevant waivers or amendments have been obtained and are in full force and effect, nor will any such action result in a violation of the Certificate of Incorporation or By-Laws of the Company or any law or any order, decree, rule or regulation of any court or governmental agency having jurisdiction over the Company or its properties.

(h) The Trust is not, and, upon giving effect to the transactions contemplated hereby, will not be required to register as an “investment company” under the 1940 Act, as amended.

(i) Assuming the Initial Purchasers’ representations are true and accurate, it is not necessary in connection with the offer, sale and delivery of the Offered Notes in the manner contemplated by this Agreement and the Final Memorandum to register the Offered Notes under the Securities Act or to qualify the Indenture under the Trust Indenture Act of 1939, as amended.

(j) The Offered Notes satisfy the requirements set forth in Rule 144A(d)(3) under the Securities Act.

(k) At the time of execution and delivery of the Transfer and Servicing Agreement and after giving effect to any releases pursuant to the Warehouse Transactions, the Trust Depositor owned the Business Loans free and clear of all liens, encumbrances, adverse claims or security interests (“ Liens ”) except for Permitted Liens, and the Trust Depositor had the power and authority to transfer the Business Loans to the Trust.

(l) Upon the execution and delivery of the Transaction Documents, payment by the Initial Purchasers for the Offered Notes and delivery to the Initial Purchasers of the Offered Notes, the Trust will own the Business Loans and the Initial Purchasers will acquire title to the Offered Notes, in each case free of Liens except such Liens as may be created or granted by the Initial Purchasers and those permitted by the Transaction Documents.

(m) Assuming as to any requirements under the Securities Act only, the accuracy of the Initial Purchasers’ representations, no consent, authorization or order of, or filing or registration with, any court or governmental agency is required for the issuance and sale of the Offered Notes or the execution, delivery and performance by the Company of this Agreement or the other Transaction Documents to which it is a party, except such consents, approvals, authorizations, registrations or qualifications as have been obtained or as may be required under state securities or blue sky laws in connection with the sale and delivery of the Offered Notes in the manner contemplated herein.

(n) The Business Loans, individually and in the aggregate, have the characteristics described in each Memorandum.

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(o) Each of the representations and warranties of the Company and the Trust Depositor set forth in each of the other Transaction Documents is true and correct in all material respects.

(p) Any taxes, fees and other governmental charges payable in connection with the execution, delivery and issuance of this Agreement and the other Transaction Documents and the Notes have been or will be paid by the Company prior to the Closing Date.

(q) No adverse selection procedures were used in selecting the Business Loans from among the loans that meet the representations and warranties of the Company contained in the Transfer Agreement and that are included in the Loan Pool.

(r) Neither the Company nor any affiliate thereof nor anyone acting on their behalf has, directly or indirectly (except to or through the Initial Purchasers), sold or offered, or attempted to offer or sell, or solicited any offers to buy, or otherwise approached or negotiated in respect of, any of the Offered Notes and neither the Company nor any of its affiliates will do any of the foregoing. As used herein, the terms “offer” and “sale” have the meanings specified in Section 2(3) of the Securities Act.

(s) Neither the Company nor any affiliate (as defined in Rule 501(b) of Regulation D under the Securities Act (such regulation, “ Regulation D ”)) of the Company has directly, or through any agent (other than the Initial Purchasers, as to which the Company makes no representation or warranty), sold, offered for sale, solicited offers to buy or otherwise negotiated in respect of, any security (as defined in the Securities Act) which is or may be integrated with the sale of the Offered Notes in a manner that would require the registration under the Securities Act of the offerings contemplated by the Final Memorandum or engaged in any form of general solicitation or general advertising (within the meaning of Regulation D) in connection with the offerings of the Offered Notes.

(t) With respect to any Offered Notes subject to the provisions of Regulation S, the Company has not offered or sold such Offered Notes during the Distribution Compliance Period to a person (other than any Initial Purchaser) who is within the United States or its possessions or to a United States person. For this purpose, the terms “Distribution Compliance Period”, “United States or its possessions” and “United States person” are defined as such terms are defined for purposes of Treas. Reg. § 1.163-5(c)(2)(i)(D).

The sale of the Purchased Notes to the Initial Purchasers will be made without registration of the Purchased Notes under the Securities Act, in reliance upon the exemption therefrom provided by Section 4(2) of the Securities Act.

(a) The Company and the Initial Purchasers agree that the Purchased Notes will be offered and sold only in transactions exempt from registration under the Securities Act. The Company and the Initial Purchasers will each reasonably believe at the time of any sale of the Purchased Notes by the Company through the Initial Purchasers, as initial purchasers, (i) that either (A) each purchaser of the Purchased Notes is an institutional investor that is (1) a QIB who is a Qualified Purchaser in transactions meeting the requirements of Rule 144A under the

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Securities Act and provides the Initial Purchasers with a written certification in substantially the form attached as Exhibit D-2 to the Indenture, or (2) an Institutional Accredited Investor who is a Qualified Purchaser who, in either case, purchases for its own account or for any discretionary account for which it is acquiring the Purchased Notes and provides the Initial Purchasers with a written certification in substantially the form attached as Exhibit D-1 to the Indenture, or (B) each purchaser is acquiring the Purchased Notes in an offshore transaction meeting the requirements of Regulation S, and (ii) that the offering of the Purchased Notes will be made in a manner it reasonably believes will enable the offer and sale of the Purchased Notes to be exempt from registration under state securities (or “blue sky”) laws; and each such party understands that no action has been taken to permit a public offering in any jurisdiction where action would be required for such purpose. The Company and the Initial Purchasers each further agree not to (x) engage in any activity that would constitute a public offering of the Purchased Notes within the meaning of Section 4(2) of the Securities Act or (y) offer or sell the Purchased Notes by any form of general solicitation or general advertising (as those terms are used in Regulation D), including the methods described in Rule 502(c) of Regulation D, in connection with any offer or sale of the Purchased Notes.

(b) Each Initial Purchaser hereby represents and warrants to and agrees with the Company, that (i) such Initial Purchaser is a QIB, and (ii) such Initial Purchaser will offer the Purchased Notes only (A) to persons who such Initial Purchaser reasonably believes are QIBs who are Qualified Purchasers in transactions meeting the requirements of Rule 144A, (B) to institutional investors who such Initial Purchaser reasonably believes are Institutional Accredited Investors who are Qualified Purchasers or (C) other than in the case of the Class A-2A Notes prior to the Commitment Termination Date, in offshore transactions in reliance on Regulation S. Each Initial Purchaser further agrees that it will (x) deliver to each Person who purchases Purchased Notes from such Initial Purchaser, at or prior to the confirmation of sale, a copy of the Final Memorandum, as then amended or supplemented, which Final Memorandum will include a Notice to Investors in the form attached hereto as Exhibit A , and (y) prior to any sale of the Purchased Notes to an Institutional Accredited Investor that such Initial Purchaser does not reasonably believe is a QIB, it will receive from such Institutional Accredited Investor a written certification in substantially the form attached as Exhibit D-1 to the Indenture.

(c) Each Initial Purchaser represents and warrants, solely as to itself, that it is duly authorized and possesses the requisite corporate power to enter into this Agreement.

(d) Each Initial Purchaser represents and warrants, solely as to itself, that there is no action, suit or proceeding pending against or, to the knowledge of such Initial Purchaser, threatened against or affecting, such Initial Purchaser before any court or arbitrator or any government body, agency, or official which could materially adversely affect the ability of such Initial Purchaser to perform its obligations under this Agreement.

(e) Each Initial Purchaser represents and agrees, solely as to itself, that all offers and sales of the Purchased Notes by it to non-United States persons, prior to the expiration of the Distribution Compliance Period, will be made only in accordance with the provisions of Rule 903 of Regulation S (except to the extent of any beneficial owners thereof who acquired an interest therein pursuant to another exemption from registration under the Securities Act and who will take delivery of a beneficial ownership interest in a Global Note, as contemplated in the

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Indenture) and only upon the receipt of the certification of beneficial ownership of the securities by a non-United States person in the form provided in the Indenture. For this purpose, the terms “Distribution Compliance Period” and “United States person” are defined as such terms are defined for purposes of Treas. Reg. § 1.163-5(c)(2)(i)(D).

(f) Each Initial Purchaser represents and warrants, solely as to itself, that (i) it has only communicated or caused to be communicated and will only communicate or cause to be communicated an invitation or inducement to engage in investment activity (within the meaning of Section 21 of the Financial Services and Markets Act 2000 (the “ FSMA ”)) received by it in connection with the issue or sale of any of the Notes in circumstances in which Section 21(1) of the FSMA does not apply to the Issuer; and (ii) it has complied and will comply with all applicable provisions of the FSMA with respect to anything done by it in relation to the Notes in, from or otherwise invol