<PAGE>
                                                                   Exhibit 10.23


                                                                  EXECUTION COPY

          THIRD AMENDMENT, dated as of August 23, 1999 (this "Amendment"), to
the Credit Agreement, dated as of November 13, 1997 (as amended by this
Amendment, the First Amendment, dated as of April 21, 1998, and the Second
Amendment, dated as of December 17, 1998, and as the same may be further
amended, supplemented or otherwise modified from time to time, the "Credit
Agreement"), among ASI SOLUTIONS INCORPORATED, a Delaware corporation ("ASI"),
MCLAGAN PARTNERS, INC. ("McLagan Partners", together with ASI, the "Borrowers"),
the several banks and other financial institutions from time to time parties
thereto (collectively, the "Lenders"), and THE CHASE MANHATTAN BANK, a New York
banking corporation, as administrative agent for the Lenders (in such capacity,
the "Administrative Agent").

                             W I T N E S S E T H:
                             - - - - - - - - - -

          WHEREAS, the Borrowers, the Lenders and the Administrative Agent are
parties to the Credit Agreement; and

          WHEREAS, the Borrowers have requested that the Lenders amend certain
terms in the Credit Agreement and in the manner provided for herein; and

          WHEREAS, the Administrative Agent and the Lenders are willing to agree
to the requested amendments, waivers and consents;

          NOW, THEREFORE, in consideration of the premises contained herein, the
parties hereto agree as follows:

          1.   Defined Terms.  Unless otherwise defined herein, terms which are
               -------------
defined in the Credit Agreement and used herein (and in the recitals hereto) as
defined terms are so used as so defined.

          2.   Amendment of Subsection 1.1.  Subsection 1.1 of the Credit
               ---------------------------
Agreement is hereby amended by adding the following definition in the proper
alphabetical order:

     "Long-Term Letter of Credit":  the Letter of Credit dated August 23, 1999
     issued by the Issuing Bank to this Agreement for the benefit of Teachers
     Insurance and Annuity Association of America in the initial face amount of
     $254,250.

          3.   Amendment to Subsection 3.1(b)(ii).  Subsection 3.1(b)(ii) is
               ----------------------------------
hereby amended by adding the following after the word "Date":

     "except for the Long-Term Letter of Credit, which shall expire no later
than October 31, 2006".

          4.   Representations and Warranties.  On and as of the date hereof,
               ------------------------------
the Borrowers hereby confirm, reaffirm and restate the representations and
warranties set forth in Section 4 of the Credit Agreement mutatis mutandis,
                                                          ------- --------
except to the extent that such representations and warranties expressly relate
to a specific earlier date in which case the Borrowers hereby confirm, reaffirm
and restate such representations and warranties as of such earlier date.
<PAGE>

                                                                               2



          5.   Effectiveness.  This Amendment shall become effective as of the
               -------------
date first written above upon receipt by the Administrative Agent (a)  of
counterparts of this Amendment duly executed by the Borrowers and each Lender,
(b) a Cash Collateral Agreement, substantially in the form attached hereto as
Exhibit A, duly executed by the Borrower and the Administrative Agent (the "Cash
Collateral Agreement") and (c) evidence satisfactory to the Administrative Agent
that at least $158,000 shall have been deposited in cash in the Cash Collateral
Account established pursuant to the Cash Collateral Agreement.

          6.   Continuing Effect; No Other Amendments.  Except as expressly
               --------------------------------------
provided herein, all of the terms and provisions of the Credit Agreement are and
shall remain in full force and effect.  The amendment provided for herein is
limited to the specific subsection of the Credit Agreement specified herein and
shall not constitute a consent, waiver or amendment of, or an indication of the
Administrative Agent's or the Lenders' willingness to consent to any action
requiring consent under any other provisions of the Credit Agreement or the same
subsection for any other date or time period.

          7.   Expenses.  The Borrowers agree to pay and reimburse the
               --------
Administrative Agent for all its reasonable costs and out-of-pocket expenses
incurred in connection with the preparation and delivery of this Amendment,
including, without limitation, the reasonable fees and disbursements of counsel
to the Administrative Agent.

          8.   Counterparts.  This Amendment may be executed in any number of
               ------------
counterparts by the parties hereto (including by facsimile transmission), each
of which counterparts when so executed shall be an original, but all the
counterparts shall together constitute one and the same instrument.

          9.   GOVERNING LAW.  THIS AMENDMENT SHALL BE GOVERNED
               -------------
BY, AND CONSTRUED AND INTERPRETED IN ACCORDANCE WITH, THE LAWS OF THE STATE OF
NEW YORK.
<PAGE>

                                                                               3

          IN WITNESS WHEREOF, the parties hereto have caused this Amendment to
be executed and delivered by their respective duly authorized officers as of the
date first above written.

                              ASI SOLUTIONS INCORPORATED

                              By: /s/ Michael J. Mele
                                  -----------------------------------
                                  Title: Senior Vice President & Chief
                                         Executive Officer

                              McLAGAN PARTNERS, INC.

                              By: /s/ Michael J. Mele
                                  -----------------------------------
                                  Title: Vice President

                              THE CHASE MANHATTAN BANK,
                              as Administrative Agent and as a Lender

                              By:  /s/ Stephen Zajac
                                  -----------------------------------
                                  Title: Vice President

                              STATE STREET BANK AND TRUST COMPANY,
                              as a Lender

                              By: /s/ Ralph R. Bumbaca
                                 ------------------------------------
                                 Title: Vice President

                              EUROPEAN AMERICAN BANK, as a Lender

                              By: /s/ Anthony Tomich
                                  ------------------------------------
                                  Title: Assistant Vice President

Consented To:
<PAGE>

                                                                               4

C3 SOLUTIONS INCORPORATED

By: /s/ Michael J. Mele
    ---------------------------------
    Title: Treasurer

ASSESSMENT SOLUTIONS INCORPORATED

By: /s/ Michael J. Mele
    ---------------------------------
    Title: Treasurer

PROUDFOOT REPORTS INCORPORATED

By: /s/ Michael J. Mele
    ---------------------------------
    Title: Treasurer

T3 SOLUTIONS INCORPORATED

By: /s/ Michael J. Mele
    ---------------------------------
    Title: Treasurer

MCLAGAN PARTNERS INTERNATIONAL, INC.

By: /s/ Michael J. Mele
    ---------------------------------
    Title: Treasurer

MCLAGAN PARTNERS ASIA, INC.

By: /s/ Michael J. Mele
    ---------------------------------
    Title: Treasurer

<PAGE>

                                                                       Exhibit A

          CASH COLLATERAL AGREEMENT dated as of August 23, 1999 among ASI
SOLUTIONS INCORPORATED, a Delaware corporation ("ASI"), MCLAGAN PARTNERS, INC.
("McLagan Partners", together with ASI, the "Borrowers"), and THE CHASE
MANHATTAN BANK, a New York banking corporation, as administrative agent for the
Lenders (in such capacity, the "Administrative Agent") for the Lenders parties
to the Credit Agreement, dated as of November 13, 1997 ( as amended,
supplemented or otherwise modified from time to time, the "Credit Agreement")
among the Borrowers, the Administrative Agent and such Lenders.

                             W I T N E S S E T H:
                             - - - - - - - - - -

          WHEREAS, pursuant to the Credit Agreement, the Lenders severally have
agreed to extend credit to the Borrowers upon the terms and subject to the
conditions set forth in the Credit Agreement; and

          WHEREAS, it is a condition precedent to the obligation of the
effectiveness of the Third Amendment, dated as of August 23, 1999, to the Credit
Agreement that the Borrower shall have executed and delivered to the
Administrative Agent this Agreement.

          NOW, THEREFORE, in consideration of the premises and to induce the
Issuing Bank and the Lenders to issue the Long-Term Letter of Credit under the
Credit Agreement for the account of the Borrower, the Borrower hereby agrees
with the Administrative Agent, for the benefit of the Lenders, as follows:

          1.   Defined Terms.  (a)  Unless otherwise defined herein, terms
               -------------
defined in the Credit Agreement and used herein shall have the meanings given to
them in the Credit Agreement.

          (b)  The following terms shall have the following meanings:

          "Agreement":  this Cash Collateral Agreement, as the same may be
           ---------
amended, modified or otherwise supplemented from time to time.

          "Cash Collateral":  the collective reference to:
           ---------------

          (a)  all cash, instruments, securities and funds deposited from time
to time in the Cash Collateral Account;

          (b)  all investments of funds in the Cash Collateral Account and all
instruments and securities evidencing such investments; and

          (c)  all interest, dividends, cash, instruments, securities and other
property received in respect of, or as proceeds of, or in substitution or
exchange for, any of the foregoing.
<PAGE>

                                                                               2

          "Cash Collateral Account": CD no. 209276665 established at the office
           -----------------------
of The Chase Manhattan Bank at 7600 Jericho Turnpike, Woodbury, New York 11797,
designated "The Chase Manhattan Bank - ASI Cash Collateral Account."

          "Code": the Uniform Commercial Code from time to time in effect in
           ----
the State of New York.

          "Collateral":  the collective reference to the Cash Collateral and the
           ----------
Cash Collateral Account.

          "Obligations":  as defined in the Guarantee and Collateral Agreement.
           -----------

          (c)  The words "hereof," "herein" and "hereunder" and words of similar
import when used in this Agreement shall refer to this Agreement as a whole and
not to any particular provision of this Agreement, and section and paragraph
references are to this Agreement unless otherwise specified.

          (d)  The meanings given to terms defined herein shall be equally
applicable to both the singular and plural forms of such terms.

          2.   Grant of Security Interest.  As collateral security for the
               --------------------------
prompt and complete payment and performance when due (whether at the stated
maturity, by acceleration or otherwise) of the Obligations, the Borrower hereby
grants to the Administrative Agent, for the benefit of the Administrative Agent
and the Lenders, a security interest in the Collateral.

          3.   Maintenance of Cash Collateral Account.  (a) The Cash Collateral
               --------------------------------------
Account shall be maintained until the Long-Term Letter of Credit has been
terminated.  Cash Collateral will be released to the Borrowers only in
accordance with Section 8 below.

          (b) The Collateral shall be subject to the exclusive dominion and
control of the Administrative Agent, which shall hold the Cash Collateral and
administer the Cash Collateral Account subject to the terms and conditions of
this Agreement.  The Borrower shall have no right of withdrawal from the Cash
Collateral Account nor any other right or power with respect to the Collateral,
except as expressly provided herein.  Without prejudice to any rights of the
Administrative Agent or the Lenders as specifically set forth herein or to the
right of the Administrative Agent and the Lenders to exercise other rights and
remedies against the Borrowers and other collateral and guarantees, it is the
express intent of the parties hereto that the Cash Collateral be utilized by the
Administrative Agent in accordance with the terms hereof or to satisfy the
Obligations solely in the Event of Default.

          4.   Deposit of Funds.  Upon execution and delivery of this Agreement,
               ----------------
the Borrowers shall deposit in the Cash Collateral Account immediately available
funds in the amount of at least $158,000.

          5.   Representations and Warranties. The Borrowers hereby, jointly and
               ------------------------------
severally, represent and warrant that:
<PAGE>

                                                                               3

          (a)  The Borrowers have the power and authority and the legal right to
execute and deliver, to perform their respective obligations under, and to grant
the security interest in the Collateral pursuant to, this Agreement and has
taken all necessary actions to authorize its execution, delivery and performance
of, and grant of the security interest in the Collateral pursuant to, this
Agreement.

          (b)  This Agreement constitutes a legal, valid and binding obligation
of the Borrowers enforceable in accordance with its terms and creates in favor
of the Administrative Agent a perfected, first priority security interest in the
Collateral, enforceable in accordance with its terms, except in each case as
affected by bankruptcy, insolvency, fraudulent conveyance, reorganization,
moratorium and other similar laws relating to or affecting creditors' rights
generally, general equitable principles (whether considered in a proceeding in
equity or at law) and an implied covenant of good faith and fair dealing.

          (c)  The execution, delivery and performance of this Agreement will
not violate any provision of any Requirement of Law or Contractual Obligation of
either of the Borrowers and will not result in the creation or imposition of any
Lien on any of the properties or revenues of either of the Borrowers pursuant to
any Requirement of Law or Contractual Obligation of the Borrowers, except as
contemplated hereby.

          (d)  No consent or authorization of, filing with, or other act by or
in respect of, any arbitrator or Governmental Authority and no consent of any
other Person (including, without limitation, any stockholder or creditor of the
Borrowers), is required in connection with the execution, delivery, performance,
validity or enforceability of this Agreement, other than those which have been
obtained and are in full force and effect.

          (e)  No litigation, investigation or proceeding of or before any
arbitrator or Governmental Authority is pending or, to the knowledge of either
of the Borrowers, threatened by or against the Borrowers or against any of its
properties or revenues with respect to this Agreement or any of the transactions
contemplated hereby.

          6.   Covenants.  The Borrowers covenant and agree with the
               ---------
Administrative Agent that:

          (a)  The Borrowers will not (1) sell, assign, transfer, exchange, or
otherwise dispose of, or grant any option with respect to, the Collateral, or
(2) create, incur or permit to exist any Lien or option in favor of, or any
claim of any Person with respect to, any of the Collateral, or any interest
therein, except for the security interest created by this Agreement.

          (b)  The Borrowers will maintain the security interest created by this
Agreement as a first, perfected security interest and defend the right, title
and interest of the Administrative Agent and the Lenders in and to the
Collateral against the claims and demands of all Persons whomsoever.  At any
time and from time to time, upon the written request of the Administrative
Agent, and at the sole expense of the Borrowers, the Borrowers will promptly and
duly execute and deliver such further instruments and documents and take such
further actions as the Administrative Agent reasonably may request for the
purposes of obtaining or preserving the full
<PAGE>

                                                                               4

benefits of this Agreement and of the rights and powers herein granted,
including, without limitation, of financing statements under the Uniform
Commercial Code.

          7.   Investment of Cash Collateral.  (a)  Subject to the provisions of
               -----------------------------
paragraph 7(b), collected funds on deposit in the Cash Collateral Account shall
be invested by the Administrative Agent from time to time in certificates of
deposit, time deposits or money market funds, in each case issued or offered by
The Chase Manhattan Bank or its affiliates; provided, however, that so long as
                                            --------  -------
no Default or Event of Default shall have occurred and be continuing, the
Administrative Agent shall make such investments at the direction of the
Borrowers.  All investments shall be made in the name of the Administrative
Agent or a nominee of the Administrative Agent and in a manner, determined by
the Administrative Agent in its sole discretion, that preserves the
Administrative Agent's perfected, first priority security interest in such
investments.

          (b)  The Administrative Agent shall have no obligation to invest
collected funds during the first night after their collection.

          (c)  The Administrative Agent shall have no responsibility to the
Borrowers for any loss or liability arising in respect of such investments of
the Cash Collateral (including, without limitation, as a result of the
liquidation of any thereof before maturity) or the Administrative Agent's
liquidation of any Cash Collateral to pay any Obligations, except to the extent
that such loss or liability arises from the Administrative Agent's gross
negligence or willful misconduct.

          (d)  The Borrowers will pay or reimburse the Administrative Agent for
any and all reasonable costs, expenses and liabilities of the Administrative
Agent incurred in connection with this Agreement, the maintenance and operation
of the Cash Collateral Account and the investment of the Cash Collateral,
including, without limitation, any investment, brokerage or placement
commissions and fees reasonably incurred by the Administrative Agent in
connection with the investment or reinvestment of Cash Collateral, and any
investment charges or other reasonable fees of The Chase Manhattan Bank in
connection with maintenance of the Cash Collateral Account.

          8.   Release of Cash Collateral. (a) Upon the renewal of each
               --------------------------
certificate of deposit in the Cash Collateral Account, the Administrative Agent
shall release to the Borrowers any portion of the Cash Collateral in excess of
the face value of the Long-Term Letter of Credit; provided that the
                                                  --------
Administrative Agent shall not release any such amount until the face value of
the Long-Term Letter of Credit is below $158,000.

          (b)  Upon any termination or cancellation of the Long-Term Letter of
Credit, the Administrative Agent shall release to the Borrowers, after payment
in cash of all Obligations then due and owing, all remaining Collateral in the
Cash Collateral Account.

          (c)  Upon the request of the Borrower and with the consent of each
Lender, the Administrative Agent will release to the Borrowers any portion of
the Cash Collateral in excess of the face value of the Long-Term Letter of
Credit.
<PAGE>

                                                                               5

          9.   Remedies.  (a)  Upon the occurrence of an Event of Default, the
               --------
Administrative Agent may, without notice of any kind, except for notices
required by law which may not be waived, apply the Collateral, after deducting
all reasonable costs and expenses of every kind incurred in respect thereof or
incidental to the care or safekeeping of any of the Collateral or in any way
relating to the Collateral or the rights of the Administrative Agent and the
Lenders hereunder, including, without limitation, reasonable attorneys' fees and
disbursements of counsel to the Administrative Agent, to the payment in whole or
in part of the Obligations, in such order as the Administrative Agent in its
sole discretion may elect, and only after such application and after the payment
by the Administrative Agent of any other amount required by any provision of
law, including, without limitation, Section 9-504(1)(c) of the Code, need the
Administrative Agent account for the surplus, if any, to the Borrowers.  In
addition to the rights, powers and remedies granted to it under this Agreement
and in any other agreement securing, evidencing or relating to the Obligations,
the Administrative Agent shall have all the rights, powers and remedies
available at law, including, without limitation, the rights and remedies of a
secured party under the Code. To the extent permitted by law, the Borrowers
waive presentment, demand, protest and all notices of any kind and all claims,
damages and demands it may acquire against the Administrative Agent or any
Lender arising out of the exercise by them of any rights hereunder.

          (b)  The Borrowers shall remain liable for any deficiency if the
proceeds of any sale or other disposition of the Collateral are insufficient to
pay the Obligations and the reasonable fees and disbursements of any attorneys
employed by the Administrative Agent or any Lender to collect such deficiency.

          10.  Administrative Agent's Appointment as Attorney-in-Fact.  (a)  The
               ------------------------------------------------------
Borrowers hereby irrevocably constitute and appoint the Administrative Agent and
any officer or agent of the Administrative Agent, with full power of
substitution, as its true and lawful attorney-in-fact with full irrevocable
power and authority in the place and stead of each Borrower and in the name of
each Borrower or in the Administrative Agent's own name, from time to time in
the Administrative Agent's discretion, for the purpose of carrying out the terms
of this Agreement, to take any and all appropriate action and to execute any and
all documents and instruments which may be necessary or desirable to accomplish
the purposes of this Agreement, including, without limitation, any financing
statements, endorsements, assignments or other instruments of transfer.

          (b)  The Borrowers hereby ratify all that said attorneys shall
lawfully do or cause to be done pursuant to the power of attorney granted in
Section 10(a).  All powers, authorizations and agencies contained in this
Agreement are coupled with an interest and are irrevocable until this Agreement
is terminated and the security interests created hereby are released.

          11.  Duty of Administrative Agent.  The Administrative Agent's sole
               ----------------------------
duty with respect to the custody, safekeeping and physical preservation of the
Collateral in its possession, under Section 9-207 of the Code or otherwise,
shall be to comply with the specific duties and responsibilities set forth
herein.  The powers conferred on the Administrative Agent in this Agreement are
solely for the protection of the Administrative Agent's and the Lenders'
interests in the Collateral and shall not impose any duty upon the
Administrative Agent or any Lender to exercise any such powers.  Neither the
Administrative Agent nor any Lender nor its or their
<PAGE>

                                                                               6

directors, officers, employees or agents shall be liable for any action lawfully
taken or omitted to be taken by any of them under or in connection with the
Collateral or this Agreement, except for its or their gross negligence or
willful misconduct.

          12.  Execution of Financing Statements.  Pursuant to Section 9-402 of
               ---------------------------------
the Code, the Borrowers authorize the Administrative Agent to file financing
statements with respect to the Collateral without the signature of either of the
Borrowers in such form and in such filing offices as the Administrative Agent
reasonably determines appropriate to perfect the security interests of the
Administrative Agent under this Agreement.  A carbon, photographic or other
reproduction of this Agreement shall be sufficient as a financing statement for
filing in any jurisdiction.  The Borrowers' principal place of business and
chief executive office are both located at 780 Third Avenue, New York, New York
10017.

          13.  Authority of Administrative Agent.  The Borrowers acknowledge
               ---------------------------------
that the rights and responsibilities of the Administrative Agent under this
Agreement with respect to any action taken by the Administrative Agent or the
exercise or non-exercise by the Administrative Agent of any option, right,
request, judgment or other right or remedy provided for herein or resulting or
arising out of this Agreement shall, as between the Administrative Agent and the
Lenders, be governed by the Credit Agreement and by such other agreements with
respect thereto as may exist from time to time among them, but, as between the
Administrative Agent and the Borrower, the Administrative Agent shall be
conclusively presumed to be acting as agent for the Lenders with
<PAGE>

                                                                               7

full and valid authority so to act or refrain from acting, and the Borrowers
shall not be under any obligation, or entitlement, to make any inquiry
respecting such authority.

          14.  Notices.  All notices, requests and demands to or upon the
               -------
Administrative Agent or the Borrowers to be effective shall be in writing (or by
fax confirmed in writing) and shall be deemed to have been duly given or made
(1) when delivered by hand or (2) if given by mail, when deposited in the mails
by certified mail, return receipt requested, or (3) if by fax, when sent and
receipt has been confirmed, addressed to the Administrative Agent or each
Borrower at its address or transmission number for notices provided in
subsection 10.2 of the Credit Agreement.  The Administrative Agent and the
Borrowers may change their addresses and transmission numbers for notices by
notice in the manner provided in this paragraph.

          15.  Severability.  Any provision of this Agreement which is
               ------------
prohibited or unenforceable in any jurisdiction shall, as to such jurisdiction,
be ineffective to the extent of such prohibition or unenforceability without
invalidating the remaining provisions hereof, and any such prohibition or
unenforceability in any jurisdiction shall not invalidate or render
unenforceable such provision in any other jurisdiction.

          16.  Amendments in Writing; No Waiver; Cumulative Remedies.  (a)
               -----------------------------------------------------
None of the terms or provisions of this Agreement may be waived, amended,
supplemented or otherwise modified except by a written instrument executed by
the Borrowers and the Administrative Agent, provided that any provision of this
                                            --------
Agreement may be waived by the Administrative Agent and the Lenders in a letter
or agreement executed by the Administrative Agent or by telex or facsimile
transmission from the Administrative Agent.

          (b)  Neither the Administrative Agent nor any Lender shall by any act
(except by a written instrument pursuant to Section 16(a) hereof), delay,
indulgence, omission or otherwise be deemed to have waived any right or remedy
hereunder or to have acquiesced in any Default or Event of Default or in any
breach of any of the terms and conditions hereof.  No failure to exercise, nor
any delay in exercising, on the part of the Administrative Agent or any Lender,
any right, power or privilege hereunder shall operate as a waiver thereof.  No
single or partial exercise of any right, power or privilege hereunder shall
preclude any other or further exercise thereof or the exercise of any other
right, power or privilege.  A waiver by the Administrative Agent or any Lender
of any right or remedy hereunder on any one occasion shall not be construed as a
bar to any right or remedy which the Administrative Agent or such Lender would
otherwise have on any future occasion.

     (c)  The rights and remedies herein provided are cumulative, may be
exercised singly or concurrently and are not exclusive of any other rights or
remedies provided by law.

          17.  Section Headings.  The section headings used in this Agreement
               ----------------
are for convenience of reference only and are not to affect the construction
hereof or be taken into consideration in the interpretation hereof.
<PAGE>

                                                                               8

          18.  Successors and Assigns.  This Agreement shall be binding upon the
               ----------------------
successors and assigns of the Borrowers and shall inure to the benefit of the
Administrative Agent and the Lenders and their successors and assigns.

          19.  Governing Law.  This Agreement shall be governed by, and
               -------------
construed and interpreted in accordance with, the law of the State of New York.
<PAGE>

                                                                               9

          IN WITNESS WHEREOF, the Borrowers and the Administrative Agent have
caused this Cash Collateral Agreement to be duly executed and delivered as of
the date first above written.

                              ASI SOLUTIONS INCORPORATED

                              By: /s/ Michael J. Mele
                                  ----------------------------------------
                                  Title: Senior Vice President & Chief
                                         Financial Officer

                              McLAGAN PARTNERS, INC.

                              By: /s/ Michael J. Mele
                                  ----------------------------------------
                                  Title: Treasurer

                              THE CHASE MANHATTAN BANK,
                              as Administrative Agent and as a Lender

                              By: /s/ Stephen Zajac
                                  ----------------------------------------
                                  Title: Vice President