<PAGE>   1
                                                                    EXHIBIT 10.3

                            ASSIGNMENT AND ASSUMPTION
                                       OF
                           PURCHASE AND SALE AGREEMENT

         THIS ASSIGNMENT AND ASSUMPTION OF CONTRACT OF SALE (the "Assignment")
is dated as of May 12, 2000, by and Between DONALD J. CARTER, JR. (the
"Assignor") and 1400 LAVON, L.P., a Texas limited partnership ("Assignee").

         WHEREAS, Assignor entered into that certain Purchase and Sale Agreement
effective March 16, 2000, by and between Homco, Inc., a Texas corporation, as
Seller, and Assignor, as Purchaser therein (collectively, the "Contract").

         WHEREAS Assignor desires to assign, transfer, set over and deliver to
Assignee all of Assignor's right, title and interest in and to the Contract as
hereinafter provided; and

         WHEREAS, Assignee desires to assume the duties and obligations of
Assignor under the Contract as hereinafter provided.

         NOW, THEREFORE, in accordance with the terms of this Assignment and in
consideration of the sum of Ten and no/100 Dollars ($10.00), the sufficiency and
receipt of which are hereby acknowledged, Assignor and Assignee do hereby
covenant and agree as follows:

         1. Assignor does hereby assign, transfer, set over and deliver to
Assignee, effective as of the date of this Assignment, all of Assignor's right,
title and interest in and to (a) the Contract, and (b) the Earnest Money Deposit
(as defined in the Contract) made pursuant to the Contract.

         2. Assignee hereby accepts this Assignment and hereby agrees to assume
and perform each and every obligation and duty of Assignor under the Agreement,
all with the same force and effect as if Assignee had signed the Agreement
originally as the Buyer named therein.

         3. Assignor and Assignee hereby represent that Assignee is an entity
owned and controlled by Assignor or an affiliate of Assignor.

         4. This Assignment embodies the entire agreement of Assignor and
Assignee with respect to the subject matter of this Assignment and supersedes
any prior agreements, whether written or oral, with respect to the subject
matter of this Assignment. There are no agreement or understandings which are
not set forth in this Assignment.

         5. This Assignment shall be (a) binding upon, and inure to the benefit
of, the parties to this Assignment and their respective heirs, legal
representatives, successors and assigns, and (b) construed in accordance with
the laws of the State of Texas.



<PAGE>   2



         IN WITNESS WHEREOF, this Assignment has been signed, sealed and
delivered by the parties as of the date first above written.

                                               /s/ DONALD J. CARTER, JR.
                                               ---------------------------------
                                               DONALD J. CARTER, JR.



                                               1400 LAVON, L.P.,a Texas limited
                                               partnership

                                               By: One Straight Arrow, Inc., a
                                                   Texas corporation,
                                                   general partner

                                                   By: /s/ DONALD J. CARTER, JR.
                                                       -------------------------
                                                   Name: Donald J. Carter, Jr.
                                                         -----------------------
                                                   Title: President
                                                          ----------------------