<PAGE>   1


                                                                   Exhibit 10.18

                               EIGHTH AMENDMENT TO
                    WAREHOUSING CREDIT AND SECURITY AGREEMENT
                        AND AMENDMENT OF PROMISSORY NOTE

         This Eighth Amendment to Warehousing Credit and Security Agreement
(this "Amendment"), is entered into as of the 1st day of February, 2000, by and
between CONSTRUCTION PORTFOLIO FUNDING, INC., a Texas corporation (the
"Company"), and BANK UNITED, a federal savings bank ("Lender"). Capitalized
terms used but not defined herein have the meanings assigned to them in that
certain Warehousing Credit and Security Agreement (the "Credit Agreement") dated
effective as of December 30, 1997, by and between the Company and Lender, as the
same has been or may be amended or supplemented from time to time.

         Section 1. Recitals. The Company and Lender desire to amend the Credit
Agreement, subject to the terms and conditions of this Amendment. Therefore, The
Company and Lender hereby agree as follows, intending to be legally bound:

         Section 2. Amendment of Credit Agreement. The Credit Agreement and the
Credit Note (as hereinafter defined) are hereby amended and supplemented as
follows:

         (a) Section 1.1 of the Credit Agreement is hereby amended by the
         amendment or addition of the following definitions:

                           "Termination Date" shall mean March 31, 2000, or such
                  earlier date upon which Lender's obligation to fund shall be
                  terminated pursuant to the terms of this Agreement.

         (b) Section 2.5(a) of the Credit Agreement is deleted in its entirety,
         and the following is substituted therefor:

                  "(a) The outstanding unpaid principal amount of all Advances
                  shall be payable in full on March 31, 2000.

                  Section 3. Amendment of Promissory Note. The promissory note
         ("Credit Note") dated as of July 30, 1999, in the original principal
         amount of $30,000,000, executed by Borrower and payable to the order of
         Lender is hereby amended to provide that the entire unpaid principal
         balance of this Note plus all accrued and unpaid interest shall be due
         and payable in full on March 31, 2000. The definition of the term
         "Note" in the Credit Agreement is hereby amended to mean the Credit
         Note as amended hereby, and all renewals, extensions, modifications,
         increases, rearrangements, and replacements thereof.

         Section 3. Representations. The Company represents and warrants that
all of the representations and warranties contained in the Credit Agreement and
all instruments and documents executed pursuant thereto or contemplated thereby
are true and correct in all material respects on and as of this date.


                                                                          Page 1
<PAGE>   2

         Section 4. Continued Force and Effect. Except as specifically amended
herein, all of the terms and conditions of the Credit Agreement and all other
Loan Documents are and remain in full force and effect in accordance with their
respective terms. All of the terms used herein have the same meanings as set out
in the Credit Agreement, unless amended hereby or unless the context clearly
requires otherwise. References in the Credit Agreement to the "Agreement," the
"Loan Agreement," "hereof," "herein" and words of similar import shall be deemed
to be references to the Credit Agreement as amended hereby. Any reference in the
other Loan Documents to the "Agreement," the "Line of Credit Agreement,"
"Warehouse Agreement," or the "Loan Agreement" shall be deemed to be references
to the Credit Agreement as amended through the date hereof. Any references in
the Credit Agreement or any of the Loan Documents to the Note or the Credit Note
shall be deemed to be references to that certain promissory note ("Credit Note")
dated as of July 30, 1999, in the original principal amount of $30,000,000,
executed by Borrower and payable to the order of Lender, and all renewals,
extensions, modifications, increases, rearrangements, and replacements thereof.

          Section 5. Guarantor. NAB Asset Corporation joins in the execution of
this Amendment for, among other things, the purpose of ratifying its Guaranty
dated December 30, 1997 (the "Guaranty"), which guarantees the payment of the
indebtedness evidenced by the Credit Agreement and the Credit Note. GUARANTOR
(I) CONSENTS TO THE TERMS OF THIS AMENDMENT, (II) RATIFIES AND CONFIRMS THE
GUARANTY, WHICH IS IN FULL FORCE AND EFFECT IN ACCORDANCE WITH ITS TERMS, AND
(III) ACKNOWLEDGES THAT THE GUARANTY IS NOT SUBJECT TO ANY CLAIMS, OFFSETS,
DEFENSES, OR COUNTERCLAIMS OF ANY NATURE WHATSOEVER.

         Section 6. Representations and Release of Claims. Except as otherwise
specified herein, the terms and provisions hereof shall in no manner impair,
limit, restrict or otherwise affect the obligations of the Company, Guarantor,
or any third party to Lender, as evidenced by the Loan Documents. The Company
and Guarantor hereby acknowledge, agree, and represent that (i) the Company is
indebted to Lender pursuant to the terms of the Credit Note; (ii) the liens,
security interests and assignments created and evidenced by the Loan Documents
are, respectively, first, prior, valid and subsisting liens, security interests
and assignments against the Collateral and secure all indebtedness and
obligations of the Company to Lender under the Credit Note, the Credit
Agreement, and all other Loan Documents, as modified herein; (iii) there are no
claims or offsets against, or defenses or counterclaims to, the terms or
provisions of the Loan Documents, and the other obligations created or evidenced
by the Loan Documents; (iv) neither the Company nor the Guarantor has any
claims, offsets, defenses or counterclaims arising from any of the Lender's acts
or omissions with respect to the Loan Documents, or the Lender's performance
under the Loan Documents; (v) the representations and warranties contained in
the Loan Documents are true and correct representations and warranties of the
Company, as of the date hereof; (vi) the Company promises to pay to the order of
Lender the indebtedness evidenced by the Credit Note according to the terms
thereof; and (vii) the Company is not in default and no event has occurred
which, with the passage of time, giving of notice, or both, would constitute a
default by the Company of the Company's obligations under the terms and
provisions of the Loan Documents. In consideration of the modification of
certain provisions of the Loan Documents, all as herein provided, and the other
benefits received by the Company hereunder, the Company and Guarantor hereby
RELEASE, RELINQUISH and forever DISCHARGE Lender, its predecessors, successors,
assigns, shareholders, principals, parents, subsidiaries, agents, officers,
directors, employees, attorneys and representatives


                                                                          Page 2
<PAGE>   3

(collectively, the "Lender Released Parties"), of and from any and all claims,
demands, actions and causes of action of any and every kind or character,
whether known or unknown, present or future, which the Company has, or may have
against Lender Released Parties, arising out of or with respect to any and all
transactions relating to the Credit Agreement, the Credit Note, the Original
Note, the Guaranty, and the other Loan Documents occurring prior to the date
hereof, including any other loss, expense and/or detriment, of any kind or
character, growing out of or in any way connected with or in any way resulting
from the acts, actions or omissions of the Lender Released Parties, and
including any loss, cost or damage in connection with any breach of fiduciary
duty, breach of any duty of fair dealing, breach of competence, breach of
funding commitment, undue influence, duress, economic coercion, conflict of
interest, negligence, bad faith, malpractice, violations of the Racketeer
Influence and Corrupt Organizations Act, intentional or negligent infliction of
emotional or mental distress, tortious interference with corporate governments
or prospective business advantage, tortious interference with contractual
relations, breach of contract, deceptive trade practices, libel, slander,
conspiracy, the charging, contracting for, taking, reserving, collecting or
receiving of interest in excess of the highest lawful rate applicable to the
Loan Documents (i.e., usury), any violations of federal or state law, any
violations of federal or state banking rules, laws or regulations, including,
but not limited to, any violations of Regulation B, Equal Credit Opportunity,
bank tying act claims, any violation of the Texas Free Enterprise Antitrust Act
or any violation of federal antitrust acts.

         Section 7. Severability. In the event any one or more provisions
contained in the Credit Agreement, this Amendment, or any of the Loan Documents
should be held to be invalid, illegal or unenforceable in any respect, the
validity, enforceability and legality of the remaining provisions contained
herein and therein shall not be affected in any way or impaired thereby and
shall be enforceable in accordance with their respective terms.

         Section 8. Expenses. The Company agrees to pay all out-of-pocket costs
and expenses (including reasonable fees and expenses of legal counsel) of Lender
in connection with the preparation, operation, administration and enforcement of
this Amendment.

         Section 9. Acknowledgment. Except as amended hereby, the Company and
Guarantor ratify and confirm that the Loan Documents are and remain in full
force and effect in accordance with their respective terms and that all
Collateral is unimpaired by this Amendment and secures the payment and
performance of all indebtedness and obligations of the Company under the Credit
Note, the Credit Agreement, and all other Loan Documents, as modified hereby.
The undersigned officer of the Company executing this Amendment represents and
warrants that he has full power and authority to execute and deliver this
Amendment on behalf of the Company. The undersigned officer of the Company
represents and warrants that his execution and delivery of this Amendment has
been duly authorized, and that the resolutions and affidavits previously
delivered to Lender, in connection with the execution and delivery of the Credit
Agreement, are and remain in full force and effect and have not been altered,
amended or repealed in anywise.

         Section 10. No Waiver. The Company and Guarantor agree that no Event of
Default and no Default has been waived or remedied by the execution of this
Amendment by Lender, and any such Default or Event of Default heretofore arising
and currently continuing shall continue after the execution and delivery hereof.


                                                                          Page 3
<PAGE>   4

         Section 11. Governing Law. This Amendment shall be governed by and
construed in accordance with the laws of the State of Texas and, to the extent
applicable, by federal law.

         Section 12. Counterparts. This Amendment may be executed in any number
of counterparts and all of such counterparts taken together shall be deemed to
constitute one and the same instrument.

         SECTION 13. NO ORAL AGREEMENTS. THIS WRITTEN AMENDMENT, THE CREDIT
AGREEMENT, THE CREDIT NOTE, THE GUARANTY, AND THE OTHER LOAN DOCUMENTS, ALL AS
MODIFIED HEREBY, REPRESENT THE FINAL AGREEMENT BETWEEN THE PARTIES AND MAY NOT
BE CONTRADICTED BY EVIDENCE OF PRIOR, CONTEMPORANEOUS OR ORAL AGREEMENTS OF THE
PARTIES.

         THERE ARE NO UNWRITTEN ORAL AGREEMENTS BETWEEN THE PARTIES.

         EXECUTED and effective as of the dates first written above.


COMPANY:                          GUARANTOR:
--------                          ----------

CONSTRUCTION PORTFOLIO
FUNDING, INC.,                    NAB ASSET CORPORATION,
a Texas corporation               a Texas corporation


                                  By: /s/ Alan Ferree
By: /s/ Alan Ferree                       Alan Ferree,  Senior Vice-President

                                  LENDER:
                                  -------

                                  BANK UNITED,
                                  a federal savings bank


                                  By: /s/ Darcey Carter
                                          Darcey Carter, Regional Director,
                                          Mortgage Banker Finance


                                     Page 4