<PAGE>


                                                                   EXHIBIT 10.15

                                LETTER AGREEMENT


                                          Dated as of September 18, 2003

AmeriCredit Financial Services, Inc.
801 Cherry Street, Suite 3900
Fort Worth, Texas 76102

AmeriCredit Owner Trust 2003-1
c/o AmeriCredit Financial Services, Inc.
801 Cherry Street, Suite 3900
Fort Worth, Texas 76102

AmeriCredit Warehouse Corporation
801 Cherry Street, Suite 3900
Fort Worth, Texas 76102

Bank One Trust Company, NA
2220 Chemsearch Boulevard, Suite 150
Irving, Texas 75062

Bank One, NA
1111 Polaris Parkway, Suite 1K
Oh1-0181
Columbus, Ohio 43240

Deutsche Bank AG, New York Branch
60 Wall Street, 19th Floor
New York, New York 10005

Systems & Services Technologies, Inc.
4315 Pickett Road
St. Joseph, Missouri 64503

Ladies and Gentlemen:

                Reference is hereby made to the Sale and Servicing Agreement
dated as of March 18, 2003 (the "Sale and Servicing Agreement"), among
AmeriCredit Owner Trust 2003-1, a Delaware statutory trust (the "Issuer"),
AmeriCredit Financial Services, Inc., a Delaware corporation ("AFSI"), Bank One,
NA, a national banking association (the "Indenture Trustee"), AmeriCredit
Warehouse Corporation, a Nevada corporation (the "Depositor") and Systems and
Services Technologies, Inc. (the "Master Servicer"), a Delaware corporation. Any
capitalized term used but not defined herein shall have the meaning assigned
such term in the Sale and Servicing Agreement.

<PAGE>

                                LETTER AGREEMENT

                Reference is also hereby made to the Indenture dated as of March
18, 2003 (the "Indenture") between the Issuer and the Indenture Trustee and the
Custodian Agreement dated as of March 18, 2003 (the "Custodian Agreement") among
the Issuer, AFSI, the Indenture Trustee and Bank One Trust Company, NA (the
"Custodian").

                The parties hereto hereby agree that upon satisfaction of the
following conditions on or before September 18, 2003 (the "Conditions"):

        1.      redemption of the Notes by the Issuer by the payment of the
                Redemption Price equal to (A) $877,123,692 by the Issuer to the
                Class A Noteholder, (B) $50,000,000 by the Issuer to the Class B
                Noteholder and (C) $30,000,000 by the Issuer to the Class C
                Noteholder pursuant to Section 2.16(c) of the Indenture; and

        2.      payment of $ 23,611 to the Master Servicer as payment in full of
                all amounts due to the Master Servicer under the Basic
                Documents; and

        3.      payment of $3,333 to the Indenture Trustee as payment in full of
                all amounts due to the Indenture Trustee under the Basic
                Documents; and

        4.      payment of $0 to the Custodian as payment in full of all amounts
                due to the Custodian under the Basic Documents;

                (i) the Indenture shall be deemed by each of the parties hereto
as satisfied and discharged in accordance with Section 3.01 of the Indenture and
any right to an Officer's Certificate or Tax Opinion pursuant to such Section
3.01 shall be waived, (ii) the Sale and Servicing Agreement shall be deemed by
each of the parties hereto as terminated in accordance with Section 9.1 of the
Sale and Servicing Agreement and (iii) the other Basic Documents shall be deemed
by each of the parties hereto, pursuant to payment of the Notes in full, as
terminated in accordance with their respective terms; provided, however that
with respect to each of the Basic Documents, any rights or obligations that
survive satisfaction and discharge or termination of such Basic Document in
accordance with such Basic Document's terms, shall remain in full force and
effect (the "Surviving Rights and Obligations"). Each party hereto hereby agrees
that, upon satisfaction of the Conditions, it shall have no rights or
obligations under the Basic Documents other than the Surviving Rights and
Obligations. If the Conditions are not satisfied on or before September 18,
2003, the Basic Documents shall remain in full force and effect.

                This letter agreement shall be governed by the laws of the State
of New York, without regard to conflict of law principles, and may be executed
in any number of counterparts and by different parties hereto in separate
counterparts, each of which when so executed shall be deemed to be an original
and all of which when taken together shall constitute one and the same letter
agreement. Delivery of an executed counterpart of a signature page to this
letter agreement by facsimile shall be effective as delivery of a manually
executed counterpart of this letter agreement.

<PAGE>

                IN WITNESS WHEREOF, AmeriCredit Financial Services, Inc.,
AmeriCredit Owner Trust 2003-1, AmeriCredit Warehouse Corporation, Bank One
Trust Company, NA, Bank One, NA and Deutsche Bank AG, New York Branch, on behalf
of the Majority Noteholders, have caused their names to be signed hereto by
their respective officers thereunto duly authorized on the date first above
written.

                                         AMERICREDIT FINANCIAL SERVICES, INC.

                                         By:
                                            ------------------------------------
                                         Name:
                                         Title:

                                         AMERICREDIT OWNER TRUST 2003-1,
                                         by AmeriCredit Financial Services,
                                         Inc., as attorney-in-fact

                                         By:
                                            ------------------------------------
                                         Name:
                                         Title:

                                         AMERICREDIT WAREHOUSE CORPORATION

                                         By:
                                            ------------------------------------
                                         Name:
                                         Title:

                                         BANK ONE TRUST COMPANY, NA

                                         By:
                                            ------------------------------------
                                         Name:
                                         Title:

                                         BANK ONE, NA

                                         By:
                                            ------------------------------------
                                         Name:
                                         Title:

<PAGE>

                                LETTER AGREEMENT

                                         DEUTSCHE BANK AG, NEW YORK BRANCH,
                                         on behalf of the Majority Noteholders

                                         By:
                                            ------------------------------------
                                         Name:
                                         Title:
                                         By:
                                            ------------------------------------
                                         Name:
                                         Title:

                                         SYSTEMS & SERVICES TECHNOLOGIES, INC.

                                         By:
                                            ------------------------------------
                                         Name:
                                         Title: