<PAGE>

                                                                   EXHIBIT 99-28

                 FIFTEENTH AMENDMENT TO TRUST AGREEMENT BETWEEN
                      FIDELITY MANAGEMENT TRUST COMPANY AND
                               DTE ENERGY COMPANY

      THIS FIFTEENTH AMENDMENT, dated as of the first day of January, 2002, and
effective as noted below, by and between Fidelity Management Trust Company (the
"Trustee") and DTE Energy Company (the "Sponsor");

                                   WTTNESSETH:

      WHEREAS, the Trustee and the Sponsor heretofore entered into a Master
Trust Agreement dated June 30, 1994, as amended, with regard to The Detroit
Edison Savings & Investment Plan, The Detroit Edison Savings & Investment Plan
for Employees Represented by Local 17 of the International Brotherhood of
Electrical Workers, The Detroit Edison Savings & Investment Plan for Employees
Represented by Local 223 of the Utility Workers Union of America and the MichCon
Savings and Stock Ownership Plan (collectively and individually, the "Plan");
and and

      WHEREAS, the Trustee and the Sponsor now desire to amend said Trust
Agreement as provided for in Section 14 thereof;

      NOW THEREFORE, in consideration of the above premises the Trustee and the
Sponsor hereby amend the Trust Agreement by:

      (1)   Amending Schedule "B" by adding the following:

            EXPENSE CREDIT

            Fidelity Investments Institutional Operations Company, Inc.
            (together with its affiliates, "Fidelity" for the purposes of this
            Expense Credit section) shall make available an expense credit
            ("Expense Credit") under the following terms:

                  a. As of January 1, 2002 and each January 1 thereafter, and
            based upon total Plan assets in actively-managed Fidelity Mutual
            Funds reaching $500,000,000, an Expense Credit of $110,000 per
            annum will be made available, with an additional Expense Credit of
            $75,000 per annum being made available for every additional
            $100,000,000 invested in actively-managed Fidelity Mutual Funds, on
            a book entry basis to non-interest-bearing hypothetical accounts for
            the Plans (the "Expense Credit Accounts"), to be credited to the
            Expense Credit Account of each Plan in proportion to that Plan's
            contribution to the total Plan assets in actively-managed Fidelity
            Mutual Funds.

                  b. Upon receipt of proper directions consistent with paragraph
            c hereof, the Expense Credit Account for each Plan may he debited
            and such Expense Credits used for such Plan, as follows:

                        (i) Services provided by Fidelity. Fidelity shall debit
            the Plan's Expense Credit Account, and use such Expense Credits to
            offset expenses for services

DTE Energy Company-LPS
Fifteenth Amendment
Trust Agreement

<PAGE>

      provided by Fidelity to the Plan after January 1, 2002 that would
      otherwise be payable pursuant to this Agreement as it may be amended from
      time to time.

                  (ii) Reimbursements to the Sponsor. Fidelity shall debit the
      Plan's Expense Credit Account, and shall use such Expense Credits to
      reimburse the Sponsor for direct, reasonable and necessary expenses of the
      Plan (within the meaning of ERISA) paid by the Sponsor on behalf of the
      Plan after January 1,2002.

                  (iii) No Payments to Third Parties. Fidelity shall have no
      obligation to use Expense Credits to make payments to any third party
      (i.e., any entity other than the Plan or the Sponsor) under the terms
      hereof.

            c. Directions required by this Expense Credit section shall provide
      Fidelity with instructions for payment, and shall include copies of
      invoices and a certification from the named fiduciary that (1) the Plan
      authorizes payment from its assets of expenses for which the Expense
      Credits are to be used, and could, absent the Expense Credit, be paid from
      Plan assets, (2) such expenses are reasonable, necessary and direct
      expenses of such Plan within the meaning of ERISA, and (3) the payment
      does not violate any applicable state or federal law (including without
      limitation the prohibited transaction provisions of ERISA or the Code).
      Fidelity shall have no responsibility to make or verify any certification
      provided by the named fiduciary under this paragraph. The parties
      acknowledge that reasonable, necessary and direct expenses of the Plan
      shall not include any operating expenses paid by mutual fund shareholders
      generally that are reflected in the net asset values of such mutual fund
      shares held by the Plan.

            d. Any debits or payments pursuant to paragraph b shall be limited
      to the amount of the Expense Credit Account for the Plan at the time the
      direction and appropriate documentation is submitted to Fidelity or, in
      the case of services provided by Fidelity, at the time such services would
      otherwise have been billed, and shall be subject to the Expense Credits
      Procedures attached to this Agreement as Schedule "B-l".

            e. Expense Credits cannot be used to offset or pay for any
      investment management expenses. Expense Credits cannot be used to offset,
      reimburse or pay: (i) expenses that are deducted from participant
      accounts; or (ii) expenses that are accrued in the net asset value or mil
      rate of an investment option.

            f. Any payments pursuant to subparagraph (ii) of paragraph b
      attributable to Expense Credits accruing on account of investments in
      actively-managed Fidelity Mutual Funds that have adopted a plan pursuant
      to Rule 12b-1 under the Investment Company Act of 1940 ("1940 Act") at the
      time such payments are made shall be made pursuant to such plan ("12b-1
      Payments").

                  -     The obligation to make 12b-1 Payments shall continue in
                        effect for one year from the Effective Date of this
                        Agreement, and will continue for successive annual
                        periods only upon at least annual approval by a vote of
                        the majority of the trustees for each of those Fidelity
                        Mutual Funds that have adopted such plans, including a
                        majority of those trustees that are not "interested
                        persons" (as defined in the 1940 Act) of such Mutual
                        Funds and who have no direct or indirect financial
                        interest in the operation of the plan or any agreement
                        related thereto ("Qualified Trustees").

                                       2

<PAGE>

                  -     Notwithstanding any provision of this Agreement to the
                        contrary, the obligation to make these 12b-1 Payments
                        with respect to any plan may be terminated without
                        penalty at any time, upon either a vote of a majority of
                        the Qualified Trustees, or upon a vote of a majority of
                        the outstanding voting securities (as defined in the
                        1940 Act) of the applicable Fidelity Mutual Fund to
                        terminate or not continue the plan for the applicable
                        Fidelity Mutual Fund.

                  -     Upon assignment of this Agreement, the obligation to
                        make 12b-1 Payments shall automatically terminate.

                  -     The obligation to make 12b-1 Payments shall terminate
                        upon the recipient's violation of any applicable law or
                        regulation arising from activities related to this
                        Agreement. The named fiduciary shall notify Fidelity
                        promptly if it learns of any such violation.

            Expense Credits shall expire on the first anniversary of the date
            that they were credited to the Plan's Expense Credit Account. The
            Expense Credit Account established for any Plan hereunder shall not
            be transferable under any circumstances, and shall be extinguished
            upon termination of recordkeeping services by Fidelity to such Plan,
            regardless of whether such Expense Credit Account has a hypothetical
            balance at such time. The book entry value of such account shall not
            be payable in cash to any Plan, the Sponsor, the named fiduciary or
            any other entity (including any other Plan sponsored by the
            Sponsor), except as provided in this "Expense Credit" section.

      (2)   Adding Schedule "B-1", as attached hereto,

      IN WITNESS WHEREOF, the Trustee and the Sponsor have caused this Fifteenth
Amendment to be executed by their duly authorized officers effective as of the
day and year first above written.

DTE ENERGY COMPANY                 FIDELITY MANAGEMENT TRUST
                                   COMPANY

By: /s/ Dina L. McClung            By: /s/ Sarah Mullins
    --------------------               ----------------------------------------
    Dina L. McClung                    Sarah Mullins
           Date 11/14/02               FMTC Authorized Signatory  Date 1/2/03

                                       3

<PAGE>

                                  EXHIBIT "B-1"

                            EXPENSE CREDIT PROCEDURES

The following terms govern the use of Expense Credits to defray the cost of
Fidelity-provided services or to make payments to the Plan Sponsor, the Plan or
third parties: Fidelity reserves the right to amend this Schedule B-l at any
time, upon notice to the Plan Sponsor.

-     Directions to use Expense Credits to defray Fidelity-provided expenses or
      to make payments shall be provided to Fidelity in good order on a form
      approved by Fidelity at least 45 business days before payment is due.
      Fidelity shall not be responsible to make any payments if the
      documentation is incomplete, or otherwise not in good order. Fidelity will
      notify the named fiduciary if the documentation is not in good order, but
      it shall be the responsibility of the named fiduciary to correct and
      resubmit the required documentation.

-     If the named fiduciary has directed the use of Expense Credits to defray
      the costs of Fidelity - provided services, Fidelity will apply Expense
      Credits, to the extent available, to defray such costs at the time the
      costs would otherwise be invoiced. Any charges for Fidelity-provided
      services not defrayed by Expense Credits will be due and payable pursuant
      to ordinary invoice and contract terms.

-     Fidelity will make Expense Credit payments to the extent that credits are
      available in the Plan's Expense Credit Account at the time documentation
      is submitted in good order to Fidelity Institutional Retirement Services
      Company, Finance Department, 82 Devonshire Street, Mail Zone MM3I, Boston,
      MA 02109-3614, Attention: Client Billing. A copy of all submissions should
      be provided to the Plan's relationship manager, and all inquiries
      regarding application of Expense Credits should be directed to the
      relationship manager.

-     If the submitted expenses exceed the amount of Expense Credits available
      (after costs of Fidelity-provided services have been offset, if
      applicable), Fidelity will pay amounts in the order submitted to Fidelity
      in their entirety, until Expense Credits are exhausted, and will pay a
      portion of the next-submitted amount, up to the available balance in the
      Expense Credit Account. In the event the Expense Credits are insufficient
      to pay all expenses listed, Fidelity will notify the Plan Sponsor within
      10 business days. Fidelity will not thereafter use Expense Credits
      (including later-accrued Expense Credits) to pay or defray such amounts,
      without specific direction from the named fiduciary.

Fidelity is not responsible for any late charges, interest or penalties that may
accrue owing to untimely submission to Fidelity of bills and documentation in
good order. Fidelity will use Expense Credits to defray such late charges,
interest or penalties only if expressly directed to do so.

                                       4