<PAGE>
 
                                                                   EXHIBIT 10.13

                                PROMISSORY NOTE
                                ---------------


Dallas, Texas                    $7,500,000.00                December 30, 1996


     CompUSA Inc., a Delaware corporation (the "Borrower"), for value received,
promises to pay to the order of THE SUMITOMO BANK, LIMITED ("Lender"), at the
principal office of NationsBank of Texas, N.A., in lawful money of the United
States of America, the principal sum of SEVEN MILLION FIVE HUNDRED THOUSAND AND
NO/100 DOLLARS ($7,500,000.00), or such lesser sum as shall be due and payable
from time to time hereunder, as hereinafter provided.  All terms used but not
defined herein shall have the meanings set forth in the Credit Agreement
described below.

     Principal of and interest on the unpaid principal balance of Advances under
this Note from time to time outstanding shall be due and payable as set forth in
the Credit Agreement.

     This Note is issued pursuant to and evidences Advances under an Amended and
Restated Credit Agreement, dated as of December 30, 1996, among the Borrower,
NationsBank of Texas, N.A., as Administrative Lender, and the lenders parties
thereto (as amended, restated, supplemented, renewed, extended or otherwise
modified from time to time, "Credit Agreement"), to which reference is made for
a statement of the rights and obligations of the Lender and the duties and
obligations of the Borrower in relation thereto; but neither this reference to
the Credit Agreement nor any provision thereof shall affect or impair the
absolute and unconditional obligation of the Borrower to pay the principal sum
of and interest on this Note when due.

     The Borrower and all endorsers, sureties and guarantors of this Note
(without waiving any notice that any Lender is specifically required to give
pursuant to Section 8.1 of the Credit Agreement) hereby severally waive demand,
            -----------                                                        
presentment for payment, protest, notice of protest, notice of intention to
accelerate the maturity of this Note, notice of acceleration of this Note,
diligence in collecting, the bringing of any suit against any party and any
notice of or defense on account of any extensions, renewals, partial payments or
changes in any manner of or in this Note or in any of its terms, provisions and
covenants, or any releases or substitutions of any security, or any delay,
indulgence or other act of any trustee or any holder hereof, whether before or
after maturity.

     THIS NOTE, TOGETHER WITH THE OTHER LOAN DOCUMENTS, REPRESENTS THE FINAL
AGREEMENT BETWEEN THE PARTIES AND MAY NOT BE CONTRADICTED BY EVIDENCE OF PRIOR,
CONTEMPORANEOUS, OR
<PAGE>
 
SUBSEQUENT ORAL AGREEMENTS OF THE PARTIES HERETO.  THERE ARE NO UNWRITTEN ORAL
AGREEMENTS BETWEEN THE PARTIES.

                                        CompUSA Inc.



                                        By:  /s/ Robert Gary
                                            -------------------------------
                                            Name:  Robert Gary
                                                   ------------------------
                                            Title: Vice President - Finance
                                                   ------------------------