Letter Agreement

Exhibit 10.28

[Execution Copy]

As of June 9, 2008

Mr. Laurence A. Tosi

388 W. Broadway

Loft B

New York, New York 10012

Dear Laurence:

We are pleased to confirm the terms relating to your becoming a Senior Managing Director (“ SMD ”) and Chief Financial Officer of Blackstone (as defined below) as soon as possible, but no later than December 10, 2008 (your “ Start Date ”). This letter agreement (this “ SMD Agreement” ) shall be effective as of the “as of” date first set forth above (the “ Effective Date ”) and sets forth the terms of your becoming an SMD with Blackstone (as defined below). “ Blackstone ” or “ Blackstone Entities ” means The Blackstone Group L.P. and its current and future affiliates; provided , that the terms “Blackstone” and “Blackstone Entities” do not include any investment fund affiliated with a Blackstone Entity or any portfolio company or underlying investment of any fund affiliated with a Blackstone Entity. The limited liability company agreement, limited partnership agreement or other governing agreement of any Blackstone Entity in which you have a partnership, membership or other participation interest, in each case now or hereafter in existence and as amended and/or restated, is herein called such Blackstone Entity’s “ Governing Agreement .” “ Active Member ” of a Blackstone Entity means a person who is (i) an SMD and (ii) an active member or partner (excluding a withdrawn, retaining withdrawn or deceased member or partner) of such Blackstone Entity.

1. Title; Reporting; Key Responsibilities .

(a) You will be Chief Financial Officer and an SMD of Blackstone .

(b) You will report to Hamilton E. James (“ HEJ ”), President and Chief Operating Officer of the firm, and, if HEJ is not available, to Stephen A. Schwarzman (“ SAS ”), Chairman and Chief Executive Officer of the firm.

(c) You will (i) serve on the Executive Committee of Blackstone, (ii) attend meetings of the Management Committee and (iii) have such other duties as are customary for the chief financial officer of a public company.

2. Your Annual Draw; Health and Related Benefits, Guaranteed Compensation .

(a) Except as otherwise provided herein, you will be paid such distributions and benefits as may be determined by Blackstone from time to time. Effective as of your Start Date, you will be entitled to take a draw at an annual rate equal to the annual draw of other SMDs generally (which rate currently is $350,000 (prorated for any portion of a calendar year in which you are not an SMD)), payable in equal monthly installments against your allocable share of the net pre-tax income of certain Blackstone Entities and/or your annual bonus compensation (including the 2008 Minimum Comp (defined below) for purposes of the 2008 calendar year). You understand and agree that (x) the amounts and types of your distributions remain at all times subject to the sole discretion of Blackstone and are subject to change at any time and (y) Blackstone may alter, amend, modify, discontinue or supplement any and all benefits, policies and programs at any time in its sole discretion, except in each case for 2008 Minimum Comp as provided in Section 2(d) below and your percentage of net management fee income and carried interest

for 2009 as provided in Section 2(e) below. Your annual bonus may be subject to Blackstone’s policies regarding minimum deferral on a basis generally consistent with other SMDs; provided that you shall not be required by Blackstone to defer any portion of the 2008 Minimum Comp.

(b) You will also receive health care insurance and other benefits related to such health care insurance comparable to those provided generally to all SMDs. You hereby acknowledge that, as an SMD, you will be responsible for the payment of such insurance and other benefits on the same basis generally as other SMDs. You will also receive all other benefits generally available to other SMDs, including five weeks of annual vacation (prorated for any calendar year in which you are an SMD for less than the entire calendar year).

(c) During your service at Blackstone (in any capacity) and until the expiration of all transfer restrictions applicable to any limited partner interests or units you may hold of Blackstone Holdings or The Blackstone Group L.P., respectively (collectively, the “ BX Units ”), you agree (on behalf of yourself and any and all estate planning vehicles, partnerships or other legal entities controlled by or affiliated with you (“ Affiliated Vehicles ”)) that all BX Units held by you and all such Affiliated Vehicles will only be held in an account at Blackstone’s equity plan administrator or otherwise administered by such administrator.

(d) If (i) you are still an SMD with Blackstone on December 31, 2008, (ii) Blackstone terminates your services as an SMD without Cause (as defined below, except that for purposes of this Section 2(d), Section 3(c)(ii) and Section 3(c)(iii), all determinations of Cause shall be made using an objective, reasonable person standard) prior to December 31, 2008 or (iii) Blackstone breaches its agreement to commence your service on the terms specified in Section 1 on or about your Start Date, you will be entitled to guaranteed minimum aggregate compensation of $3,500,000 cash for 2008 (“ 2008 Minimum Comp ”), which amount includes the pro rated amount of your annual draw as set forth in Section 2(a) for services performed in 2008 and your annual bonus (including any portion of such amounts that you may, at your sole discretion, elect to defer). If your service as an SMD is terminated by Blackstone without Cause prior to December 31, 2008 or Blackstone breaches its agreement to commence your service on the terms specified in Section 1 on or about the Start Date, your 2008 Minimum Comp shall be paid to you within 30 days of such termination or the elapsed Start Date, as applicable.

(e) If you are still an SMD with Blackstone on December 31, 2009, you will be entitled to (i) 0.215% of the net management fee income (before SMD compensation and taxes except New York City unincorporated business tax) from Blackstone Entities that generate (non-performance/non-incentive) management fees from investment funds and portfolio companies affiliated with any Blackstone Entities and (ii) 0.2% of the profit sharing percentage in all Blackstone carried interest vehicle transactions that close after January 1, 2009, subject to applicable vesting. Carried interest with respect to any transaction will be subject to customary obligations and conditions contained in the Governing Agreements of the Blackstone Entity (e.g., vesting, clawback, holdback, capital commitment obligations, forfeitures, future dilution) on the same terms applicable to other SMDs generally working on such transaction.

3. Side-by-Side Investment; Funds of Funds; Deferred Units .

(a) Side-by-Side Allocations . You will also be allocated $1,250,000 for each election period (or approximately $2,500,000 per year at a normalized investment rate) (subject to proration for any portion of an annual election period in which you are not an SMD) in side-by-side investment opportunities generated through BCP V, BREP VI, BREP International II and Blackstone Mezzanine Partners II L.P. (and/or other side-by-side investment opportunities that become available to SMDs generally), such aggregate amount to be allocated among the foregoing funds for each annual

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election period in proportion, as nearly as practicable, to the amounts of side-by-side investment opportunities available through the respective funds for such annual election period; provided that (i) your allocations will provide you the opportunity, but not the obligation, to invest personally and (ii) your allocations in 2008 shall be made available only if you can demonstrate to Blackstone that you are a “qualified purchaser” under the Securities Act of 1933, as amended.

(b) Funds of Funds . You will have the opportunity, but not the obligation, to invest personally in Blackstone’s fund of funds investment products managed by Blackstone Alternative Asset Management L.P. (or its successor, “ BAAM ”) as long as you serve as an SMD, subject to the same limitations on exclusions from management fees or incentive fees that are applicable to other SMDs generally with respect to their BAAM investments; provided that you invest an amount in such fund of funds investment products that is reasonably consistent with amounts invested therein generally by other SMDs, as determined by Blackstone.

(c) Deferred Unit Awards

(i) Start Date Awards. You will receive promptly after the Start Date, two equity-based awards (the “ Start Date Awards ”) under Section 8 of the Blackstone Group L.P. 2007 Equity Incentive Plan (the “ Plan ”) equal, respectively, to:

(ii) 2008 Equity Bonus Award . In addition to the Start Date Awards, if (A) you are still an SMD with Blackstone on the date in calendar year 2009 when bonuses generally are paid to other SMDs in respect of calendar year 2008 (e.g., around January 15, 2009), (B) Blackstone terminates your services as an SMD without Cause prior to such payment date or (C) Blackstone breaches its agreement to commence your service on the terms specified in Section 1 on or about your Start Date, then you will receive an equity-based award under Section 8 of the Plan equal to $4,500,000 of Deferred Units, calculated using the 30-day volume weighted average trading price of Partnership Units prior to the date of grant (the “ 2008 Equity Bonus Grant ” and, together with the Start Date Awards, the “ Awards ”).

(iii) Vesting . The Deferred Units will vest as follows:

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The unvested portion of your Awards will be terminated and will be null and void (and cease to represent a right to receive Partnership Units) once you are no longer an SMD of Blackstone for any reason, except that the then-outstanding, but unvested portion of your Awards will become fully vested (and not subject to call rights or repurchase rights, except as described in Section 3(c)(v) below or in the Non-Competition Agreement) if (i) your service with Blackstone is terminated by Blackstone without Cause or as a result of your death, permanent disability or (ii) there is a Change in Control (as defined in the partnership agreements of Blackstone Holdings). The unvested portion of your Award (and any vested Deferred Units which have been retained and not yet delivered to you pursuant to Section 3(c)(iv) below) will also be terminated and be null and void upon the termination of your service with Blackstone for Cause.

(iv) Delivery of Partnership Units . As of each vesting date with respect to your Awards, the number of Partnership Units corresponding to the vested portion of the respective Awards shall be delivered to you; provided , however , that any such Partnership Units that would otherwise be subject to continuing transfer and forfeiture restrictions as described in Section 3(c)(v) shall not be delivered to you as of the relevant vesting date and shall instead be retained by Blackstone and delivered to you as of the date on which the related transfer restrictions lapse and, in the event your services as an SMD are terminated, forfeiture restrictions lapse. Prior to the delivery of the underlying Partnership Units, you will not be the actual owner of the Partnership Units underlying your Awards and will not have any voting rights or rights with respect to distributions with respect to such Partnership Units, except as provided in Section 3(c)(vi) below.

(v) Post-Vesting Transferability and Forfeiture Restrictions. The Partnership Units delivered to you under any vested portion of your Deferred Units will generally be freely transferable after vesting (subject to the partnership agreements of Blackstone Holdings and any trading procedures established by Blackstone and generally applicable to SMDs), except that (i) you may only transfer up to 33 1/3%, 66 2/3% and 75% of the aggregate number of Deferred Units which vested under each Award on and after the first, second and third anniversaries, respectively, of your Start Date and (ii) you will not be permitted to transfer more than 75% of the aggregate number of Deferred Units which vested under your Awards during your service with Blackstone and, following your termination of service, during the non-compete and non-solicit periods described in the Non-Competition Agreement. The Partnership Units underlying your Awards which are unvested or which are not transferable and thus remain undelivered pursuant to Section 3(c)(iv), will also remain subject to the forfeiture terms described under Section 3(c)(v) hereof and Section IV of the Non-Competition Agreement until such restrictions lapse. All transfer restrictions will lapse upon (i) your death or permanent disability or (ii) a Change in Control.

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(vi) Dividends . To the extent that distributions or dividends are made with respect to Partnership Units underlying your Deferred Units (i.e., those Partnership Units that have not yet been delivered to you), you will receive such distributions or dividends with respect to those Deferred Units (whether or not vested and/or delivered) related to Blackstone earnings accruing during the periods you are associated with Blackstone; provided that you will not be entitled to dividends if you do not hold your vested units on any record date (due to for example, your selling such units prior to a record date).

4. Compliance Policies; Non-Competition.

(a) You acknowledge and agree that you are subject to all applicable provisions of the Blackstone compliance policies applicable to you that have been made available to you, including the Compliance Policies and Procedures Manual (to the extent applicable to you), Investment Adviser Compliance Policies and Procedures and its related supplements (each, to the extent applicable to you), and USA Patriot Act Anti-Money Laundering Policies, as well as Blackstone’s Code of Business Conduct and Ethics (including the Code of Ethics for Financial Professionals, if applicable) and the Employee Handbook and Business Continuity Plan (or in the case of UK-based SMDs, the U.K. AML Manual and U.K. Compliance Manual) (collectively, the “ Blackstone Compliance Policies ”).

(b) You acknowledge that you have executed the SMD Non-Competition and Non-Solicitation Agreement, attached hereto as Schedule B (the “ Non-Competition Agreement ”), and agree that the terms thereof are incorporated herein by reference.

(c) Subject to Section 5.5 of the Agreement of Limited Partnership of The Blackstone Group L.P., you acknowledge and agree that becoming a party to this SMD Agreement does not afford you any rights with respect to the management and/or operation of Blackstone.

5. Termination; Resignation

(a) You acknowledge and agree that Blackstone may terminate your service at any time for any reason, or for no reason at all with or without Cause; provided , however, that Blackstone shall provide you with written notice at least ninety days prior to the date of the termination of your service during which Blackstone may elect to place you on paid leave for all or part of such ninety-day period; provided, further, that during such ninety-day period, you shall continue to receive your base draw and benefits, subject to applicable law and the payment of benefits-related premiums, but shall not receive or participate in any profit sharing or bonus arrangements (including participation in any carried interest or fee-sharing program).

(b) Notwithstanding the foregoing, you acknowledge and agree that Blackstone may terminate your services hereunder for Cause and such termination shall be effective immediately. For purposes of this SMD Agreement, Cause means the occurrence or existence of any of the following as determined fairly, reasonably, on an informed basis and in good faith by Blackstone: (i) (w) any breach by you of any provision of the Non-Competition Agreement, (x) any material breach of any rules or regulations of Blackstone applicable to you, (y) your deliberate failure to perform your duties to Blackstone, or (z) your committing to or engaging in any conduct or behavior that is or may be harmful to Blackstone in a material way; provided that, in the case of any of the foregoing clauses (w), (x), (y) and (z), Blackstone has given you written notice (a “ Notice of Breach ”) within fifteen days after Blackstone becomes aware of such action and you fail to cure such breach, failure to perform or conduct or behavior within fifteen days after receipt by you of such Notice of Breach from Blackstone (or such longer period, not to exceed an additional fifteen days, as shall be reasonably required for such cure, provided that you are diligently pursuing such cure); (ii) any act of fraud, misappropriation, dishonesty, embezzlement or

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similar conduct against Blackstone; or (iii) conviction (on the basis of a trial or by an accepted plea of guilty or nolo contendere ) of a felony or crime (including any misdemeanor charge involving moral turpitude, false statements or misleading omissions, forgery, wrongful taking, embezzlement, extortion or bribery), or a determination by a court of competent jurisdiction, by a regulatory body or by a self-regulatory body having authority with respect to securities laws, rules or regulations of the applicable securities industry, that you individually have violated any applicable securities laws or any rules or regulations thereunder, or any rules of any such self-regulatory body (including, without limitation, any licensing requirement), if such conviction or determination has a material adverse effect on (A) your ability to function as an SMD, taking into account the services required of you and the nature of Blackstone’s business or (B) the business of Blackstone.

(c) You agree to provide Blackstone with written notice of your intention to terminate your service with Blackstone at least ninety days prior to the date of such termination (the “ Notice Period ”). Written notice pursuant to this Section 5(c) shall be provided to any of the Chief Executive Officer, Chief Operating Officer or Chief Legal Officer of Blackstone. During the Notice Period, you shall perform any and all duties as directed by Blackstone, in its sole discretion.

(d) You shall be placed on garden leave status for a period commencing on the day following the conclusion of the ninety-day Notice Period and continuing for ninety days thereafter (the “ Garden Leave Period ”). During the Garden Leave Period, you shall continue to receive your base draw and benefits, subject to the payment of related premiums, but shall not receive or participate in any profit sharing or bonus arrangements (including participation in any carried interest or fee-sharing program). During the Garden Leave Period, you shall not be required to carry out any duties for or on behalf of Blackstone. You agree that you will not enter into any employment or other business relationship with any other employer or otherwise prior to the conclusion of the Garden Leave Period.

(e) The provisions of Sections 5(c) and 5(d) shall not be applicable in instances in which your service with Blackstone is terminated by Blackstone with or without Cause (including if Blackstone elects to terminate your service during the Garden Leave Period).

6. Representations, Warranties and Covenants .

(a) Except as set forth on Schedule 6(a) and except for the agreements, arrangements and plans listed on Schedule 6(a) hereto (true, correct and complete copies of which have been delivered to Blackstone prior to the date hereof), you represent and warrant, as of the date hereof and (except as would not materially impair your ability to perform your duties as described herein) as of your Start Date, as follows:

(i) This SMD Agreement and the Non-Competition Agreement (this SMD Agreement and the Non-Competition Agreement being herein collectively called the “ Other Business Agreements ”), constitute valid and binding obligations of you and your affiliates, enforceable against you or any of your affiliates in accordance with the terms thereof.

(ii) You are not subject to:

1. any restrictive covenants, including without limitation, relating to competition, solicitation or confidentiality (other than general obligations to maintain confidentiality consistent with your fiduciary and other executive duties), arising from any agreement, oral, written or otherwise, between you and any Other Person (as defined below); or

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2. any agreement, oral, written or otherwise, between you and any Other Person, or any common law, statutory or fiduciary duty owed to any Other Person, that will in any way (I) materially compromise, limit or restrict your ability to perform your duties commencing on your Start Date on behalf of all Blackstone Entities pursuant to any Other Business Agreement, (II) purport to bind contractually or otherwise any of the Blackstone Entities, or (III) subject any Blackstone Entity (or any partner, member, affiliate, officer or employee of any of the foregoing) to any liability of any kind or to any claim by any Other Person.

“ Other Person ” means any corporation, partnership, limited liability company, sole proprietorship or other person, entity or association (other than a Blackstone Entity), including, without limitation, any Employer-affiliated Entity (as hereinafter defined). “ Past or Present Employer ” means any corporation, partnership, limited liability company, sole proprietorship or other person, entity or association with which you have or have had any employment, partnership, limited liability company, consulting or similar business relationship or of which you are or have been an officer or director. “ Employer-affiliated Entity ” means, collectively, any Past or Present Employer and any corporation, partnership, limited liability company, sole proprietorship or other person, entity or association that is an affiliate, subsidiary, predecessor or successor of any Past or Present Employer.

(iii) None of (A) the execution, delivery and performance of any of the Other Business Agreements, (B) the consummation of