<PAGE>

                                                                   EXHIBIT 10.15


                              ZIFF DAVIS MEDIA INC.
                      c/o Willis Stein & Partners II, L.P.
                             227 West Monroe Street
                                Chicago, IL 60606


                                  April 5, 2000


ZD Inc.
28 East 28th Street
New York, NY 10016
Attn:  J. Malcolm Morris, Esq.

Gentlemen:

     Reference is made to that certain Purchase Agreement (the "Purchase
Agreement") dated as of December 6, 1999 by and among ZD Inc. ("Asset Seller"),
ZD Holdings (Europe) Ltd. ("Stock Seller" and together with Asset Seller, the
"Sellers") and Ziff Davis Media Inc. (formerly WS-ZD Acquisition Inc.)
("Buyer"). Capitalized terms used in this letter agreement (the "Letter
Agreement") and not otherwise defined shall have the meanings ascribed to such
terms in the Purchase Agreement.

     On the date of this Letter Agreement, the parties hereto are effecting the
Closing of the transactions contemplated by the Purchase Agreement. In
connection with the Closing, and as a condition thereto, the parties hereto have
made certain other agreements and arrangements which the parties wish to set
forth in this Letter Agreement. Sellers and Buyer have agreed as follows:

1.   Pursuant to a letter agreement (the "Dolce Letter Agreement") dated as of
     January 1, 1998, between Jack Dolce and Asset Seller, Jack Dolce holds a
     ten percent (10%) shadow equity interest ("Dolce Equity Interest") in Asset
     Seller's publication Smart Reseller. The Dolce Letter Agreement grants Mr.
     Dolce the right to put the Dolce Equity Interest to Asset Seller under
     certain conditions, including in the event he is terminated without cause.
     The Dolce Letter Agreement also grants Asset Seller the right to call the
     Dolce Equity Interest under certain conditions, including in the event
     Asset Seller sells Smart Reseller, either independently or as part of a
     transaction involving more than just the sale of Smart Reseller. Buyer and
     Asset Seller hereby agree that, notwithstanding anything in the Purchase
     Agreement to the contrary, Asset Seller shall not assign to Buyer, and
     Buyer shall not assume from Asset Seller, the Dolce Letter Agreement as
     part of the Closing and Asset Seller shall, subject to Buyer's
     reimbursement obligations set forth below, remain responsible for all
     obligations and liabilities thereunder.
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April 5, 2000
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     On March 31, 2001, Dolce's employment with Asset Seller was terminated. Mr.
     Dolce has indicated his intent to exercise his right to put the Dolce
     Equity Interest to Asset Seller pursuant to the Dolce Letter Agreement. In
     the event Mr. Dolce does not exercise his right to put the Dolce Equity
     Interest to Asset Seller, Asset Seller hereby confirms that it will call
     the Dolce Equity Interest in a timely manner pursuant to its rights to do
     so under the Dolce Letter Agreement.

     Buyer agrees that it will reimburse Asset Seller the amount of any
     severance, vacation and incentive compensation payments it makes to Mr.
     Dolce to the extent that Asset Seller was required to make such payments to
     Mr. Dolce under terms of his employment as previously disclosed to Buyer
     (but in no event shall Buyer be obligated to reimburse Asset Seller in
     excess of $233,000 for such severance, vacation and incentive compensation
     obligations) as well as any other out-of-pocket costs Asset Seller may
     reasonably incur as a direct result of the termination of Mr. Dolce. Buyer
     further agrees to reimburse Asset Seller for the purchase price paid by
     Asset Seller to Mr. Dolce for the Dolce Equity Interest pursuant to the
     terms of the Dolce Letter Agreement as in effect in the form previously
     delivered to Buyer, which Asset Seller estimates to be the amount of
     $1,995,100. Asset Seller shall allow Buyer to participate fully in the
     negotiations with Mr. Dolce concerning the purchase of the Dolce Equity
     Interest and shall not agree to a determination of the value of Smart
     Reseller or of Mr. Dolce's interest therein, or the amount of any payment
     to Mr. Dolce, without the prior written consent of Buyer, which consent
     Buyer shall not withhold unreasonably. The amount of each reimbursement
     payment by Buyer pursuant to this paragraph 1 shall be paid immediately
     prior to or concurrently with any payment made by Asset Seller to Mr. Dolce
     which is subject to reimbursement hereunder. Notwithstanding anything in
     the Purchase Agreement to the contrary, other than a liability accrued in
     accordance with GAAP for the purchase price for the Dolce Equity Interest
     pursuant to the terms of the Dolce Letter Agreement, none of the amounts
     set forth above shall be included in the calculation of the Closing Date
     Tangible Net Worth under Section 2.8 of the Purchase Agreement.

2.   Concurrently with the Closing, Buyer will reimburse Asset Seller for an
     aggregate of $400,268.78 of information technology related expenses which
     have been incurred by Asset Seller prior to the Closing at the request of
     Buyer in connection with the transactions contemplated by the Purchase
     Agreement and which were primarily for the benefit of Buyer.
     Notwithstanding anything in the Purchase Agreement to the contrary, neither
     the asset nor the liability in respect of such information technology
     related
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April 5, 2000
Page 3

     expenses shall be included in the calculation of the Closing Date Tangible
     Net Worth under Section 2.8 of the Purchase Agreement.

3.   Asset Seller has informed Buyer that, prior to Closing, it will withdraw
     all cash from the respective accounts of each of its European subsidiaries
     (the "European Subsidiaries"). As set forth in the Purchase Agreement,
     Buyer is acquiring the stock of the European Subsidiaries as a result of
     the transactions contemplated therein. For logistical reasons, it will be
     difficult for Buyer to provide the newly acquired European Subsidiaries
     with their required cash needs for several days after the Closing Date.
     Accordingly, Asset Seller has agreed to fund each of the European
     Subsidiaries (such funding to be accomplished by leaving an amount of cash
     in each European Subsidiary) with an amount of cash requested by Buyer (the
     "Cash Flow Amount"). Such Cash Flow Amount will be $631,334 for the
     European Subsidiary located in Germany, $491,520 for the European
     Subsidiary located in United Kingdom and $181,835 for the European
     Subsidiary located in France (or $1,304,689 in the aggregate). Buyer will
     make a payment to Asset Seller at the Closing in an amount equal to the
     lesser of (i) the aggregate amount of cash held by the European
     Subsidiaries at the Closing, and (ii) the Cash Flow Amount. Notwithstanding
     anything in the Purchase Agreement to the contrary, Closing Date Tangible
     Net Worth shall be calculated under Section 2.8 of the Purchase Agreement
     assuming that Asset Seller had withdrawn all cash from the European
     Subsidiaries, and consequently, no cash held by any of the European
     Subsidiaries at the Closing shall be included in the calculation of the
     Closing Date Tangible Net Worth under Section 2.8 of the Purchase
     Agreement.

4.   Buyer requested that the employment of certain employees of the Division be
     terminated prior to the Closing, which employees are listed on the attached
     Schedule 1 (the "Terminated Employees"). In connection therewith, Buyer
     agreed that it would reimburse Asset Seller for all payments of severance
     and other benefits and obligations that Asset Seller incurred in connection
     with the termination of such individuals to the extent that Asset Seller
     was required to make such payments under the terms of the employment of
     such individuals previously disclosed to Buyer as well as any other
     out-of-pocket costs Asset Seller may reasonably incur as a direct result of
     the termination of such Terminated Employees. The attached Schedule 1 sets
     forth the calculation of the amount for which Buyer is obligated to
     reimburse Asset Seller pursuant to this paragraph 4 in respect of such
     payments of severance and other benefits and obligations (which were paid
     by Asset Seller prior to the Closing Date) which amount shall be paid by
     Buyer on the date hereof by wire transfer to an account designated by Asset
     Seller. For purposes of the determination of the Closing Date Tangible Net
     Worth, the cost of the
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April 5, 2000
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     severance and other employee benefits obligations as set forth on the
     attached Schedule 1 shall not be included as a liability as of the Closing
     Date, however, an adjustment shall be made to the Closing Date Tangible Net
     Worth calculation to reflect the liability or expense that would have been
     accrued by the Division as of the Closing Date in respect of the
     compensation, employee benefits and other employment-related expenses had
     such Terminated Employees been employed by the Division as of the Closing
     Date.

5.   As of the date of this Letter Agreement, Buyer and Sellers have been unable
     to agree upon a reasonable allocation of the purchase price between the
     Transferred Assets and the Shares of each of the respective Companies, as
     contemplated by the Purchase Agreement. In order to resolve the parties'
     dispute concerning such allocation, the parties have agreed to submit such
     dispute for resolution to an independent valuation expert mutually
     agreeable to Asset Seller and Buyer; provided, however, that if Asset
     Seller and Buyer are unable to agree upon the selection of an independent
     valuation expert within 30 days of the date of this Letter Agreement, then
     within 10 business days after the expiration of such 30-day period, each of
     Buyer and Asset Seller shall designate one independent valuation expert to
     select, within a further 10-business day period, a mutually satisfactory
     third valuation expert who shall resolve such dispute. Each of the
     valuation experts selected or designated hereunder shall be experienced in
     valuing businesses in the magazine publishing industry. The valuation
     expert so selected or designated shall be directed by the parties to
     determine the allocation of the purchase price between the Transferred
     Assets and the Shares of each of the respective Companies based upon the
     fair market value of the assets and liabilities transferred and consistent
     with the principles of Section 1060 of the Code. Each party shall have no
     longer than one day to present its position, the entire proceedings before
     the appraiser shall be on no more than three consecutive days, and the
     determination shall be made in writing no more than 30 days following the
     end of the proceeding. Buyer shall not take a position in the appraisal
     proceeding which allocates more than $60,000,000, in the aggregate, to the
     purchase price for the Shares, and Seller shall not take a position in the
     appraisal proceeding which allocates less than $40,000,000, in the
     aggregate, to the purchase price for the Shares. Such determination shall
     be a final and binding determination of the dispute. The allocation of the
     Consideration as contemplated by Section 5.14(c) of the Purchase Agreement
     shall be delayed pending outcome of the appraisal proceeding. Upon the
     final determination of the purchase price allocation as described above,
     the parties shall commence the procedures set forth in Section 5.14(c) with
     respect to the allocation of the Consideration. Pending the resolution of
     the allocation of the purchase price as described above, the purchase price
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April 5, 2000
Page 5

     paid by Buyer to the account designated by Sellers (as set forth in the
     Flow of Funds Memorandum prepared by the parties at the Closing) shall be
     deemed payment in full of the Purchase Price for Shares and the Initial
     Cash Purchase Price for the Assets, and Buyer shall have no further
     liability or obligation with respect thereto notwithstanding the future
     allocation of such payment between Asset Seller and Stock Seller once the
     Fair Market Value is finally determined under this Letter Agreement. The
     fees and costs of the appraiser shall be paid by the parties on a
     proportionate basis based upon the relative deviations or the parties'
     proposed purchase price allocations from the allocation of the purchase
     price determined by the appraiser in the above-described proceeding. Each
     of the parties shall bear their own fees and expenses (including, without
     limitation legal fees and expenses) incurred in connection with the above
     described appraisal proceeding.

     This Letter Agreement may be executed in counterparts (including by means
of telecopied signature pages), any one of which need not contain the signatures
of more than one party, but all such counterparts taken together shall
constitute one and the same agreement.
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April 5, 2000
Page 6

     IN WITNESS WHEREOF, this Letter Agreement has been executed by a duly
authorized representative of each of the parties hereto as of the date first
above written.

                                             ZIFF DAVIS MEDIA INC.

                                             By:     /s/ DANIEL H. BLUMENTHAL
                                                     ---------------------------
                                             Name:   Daniel H. Blumenthal
                                             Title:  Vice President


                                             ZD INC.

                                             By:     /s/ J. MALCOLM MORRIS
                                                     ---------------------------
                                             Name:   J. Malcolm Morris
                                             Title:  Senior Vice President


                                             ZD HOLDINGS (EUROPE) LTD.

                                             By:     /s/ J. MALCOLM MORRIS
                                                     ---------------------------
                                             Name:
                                             Title:  For Eric Hippeau
                                                     By Power of Attorney
<PAGE>

                                   Schedule 1

<TABLE>
<CAPTION>
                                                                           Enhanced                      Outplacement
                                                                           --------                      ------------
            Name                             Dept          Term Date       Severance     Vacation Pay        Fee
            ----                             ----          ---------       ---------     ------------        ---
<S>   <C>                                <C>                <C>             <C>             <C>             <C>
1     Nancy Newman                       Corp Sales         03/06/00        242,308         26,923           --

2     Kaila S Bongiovanni                Corp Sales         02/29/00         11,948          1,803           --

3     Daniel P Daly                      Corp Sales         02/29/00         27,600          6,900          3,000

4     Christopher Harding                Corp Sales         01/31/00         19,615          4,904           --

5     Karina Montgomery                  Corp Sales         02/29/00         25,479          3,004           --

6     Scott S Murphy                     Corp Sales         02/29/00         67,692         11,692           --

                                         TOTAL                              394,642         55,226          3,000

                                         GRAND TOTAL                        452,868
</TABLE>