Document:

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                                                                  EXHIBIT 10.43

                             THE UNIMARK GROUP, INC.
                                124 MCMAKIN ROAD
                            BARTONVILLE, TEXAS 76226

                                 April 17, 2000

Promecap, S.C.
Bosque de Alisos No. 47A, 3er piso
Colonia Bosques de las Lomas
C.P. 05120 Mexico, D.F.
Mexico
Attention: Fernando Chico Pardo

     RE:  The UniMark Group, Inc.-- Stand-by Funding Commitment

Dear Fernando:

         This letter confirms the firm commitment of Promecap, S.C., either
directly or through one or more affiliated or related entities (collectively,
"Promecap"), to purchase, at the election of The UniMark Group, Inc.
("UniMark"), on or before July 1, 2000, $2,500,000 of convertible subordinated
debentures (the "Convertible Debenture"), subject only to the terms and
conditions outlined below.

1.   Terms of the Convertible Debentures. The terms of the Convertible
     Debentures are outlined in Exhibit"A" hereto.

2.   Subscription Agreement. Promecap's purchase of the Convertible Debenture
     will be consummated pursuant to a Subscription Agreement with UniMark,
     which will contain customary terms and provisions.

3.   Consents. It is understood that Promecap's obligation to purchase the
     Convertible Debenture will be subject only to UniMark receiving the
     necessary consents, approvals and authorizations required in connection
     with the issuance of the Convertible Debentures and subject to paragraph 4
     below.

4.   Termination of Funding Commitment. This Agreement shall automatically
     terminate upon the earlier of:

          a)   If, prior to July 1, 2000, or the purchase of the Convertible
               Debenture as provided herein, there shall be (i) a merger or
               consolidation of UniMark, or either of UniMark's two primary
               subsidiaries, with or into another corporation, (ii) the sale of
               all, or substantially all, of the properties or assets or stock
               of either of UniMark's two primary business segments, or (iii) a
               joint venture with any other entity involving all or
               substantially all of the assets of, or revenue from, either of
               UniMark's two primary business segments (collectively, an
               "Extraordinary Transaction"); or

          b)   if, prior to July 1, 2000 or the purchase of the Convertible
               Debenture as provided herein, UniMark sells equity securities
               (common stock, preferred stock, convertible debt securities or
               subordinated debt) with gross proceeds to UniMark in excess of
               $2.5 million (a "Significant Equity Financing"); or

<PAGE>   2

          c)   if, UniMark refinances all of its outstanding indebtedness with
               Cooperatieve Centrale Raiffeisen-Boerenleenbank, B.A., "RaboBank
               Nederland," New York Branch ("Refinancing").

5.   Closing: In the event that neither an Extraordinary Transaction, a
     Significant Equity Financing nor a Refinancing has occurred on or before
     May 31, 2000, the purchase and sale of the Debenture shall occur within 5
     business days after UniMark gives Promecap written notice of its election
     to sell the Convertible Debenture to Promecap. It is agreed that UniMark
     will not give such notice until May 31, 2000.

6.   No Obligation To Sell: It is expressly understood and agreed that UniMark,
     in its sole discretion, may elect not to sell the Convertible Debenture to
     Promecap and nothing set forth herein creates an obligation on behalf of
     UniMark to sell the Convertible Debenture Promecap.

7.   Facility Fee: UniMark will pay Promecap a facility fee of $100,000 as
     consideration for facilitating this Stand-by Funding Commitment, out of
     available and unrestricted cash flow.

8.   Governing Law: This Agreement will be governed by and construed in
     accordance with the laws of the State of Texas, without regard to
     principles of conflicts of laws.

9.   Counterparts: This Agreement may be executed by the parties hereto in one
     or more counterparts, and such counterparts, taken together, shall
     constitute one and the same agreement of such parties. Any facsimile
     signature of any party will constitute an original.

     If the foregoing is acceptable to you, please execute and return the
enclosed copy of this Agreement to me by 5:00 P.M. on Monday, April 17, 2000.

                                                Very truly yours,

                                                THE UNIMARK GROUP, INC.

                                                By: Soren Bjorn
                                                    Chief Executive Officer

AGREED AND ACCEPTED AS OF
THE DATE FIRST ABOVE WRITTEN:

PROMECAP, S.C.

By:
 Fernando Chico Pardo
  Title: President

                                       2
<PAGE>   3

                                    EXHIBIT A
                                    ---------

                  CONVERTIBLE SUBORDINATED DEBENTURE TERM SHEET

<TABLE>

<S>                                 <C>
Issuer:                             The UniMark Group, Inc. (Nasdaq NMS: UNMG) ("UniMark").

Issue:                              $2,500,000  principal  amount of 12%  Convertible  Subordinated  Debenture
                                    (the "Debenture").

Closing Date:                       As  soon as  possible  after  UniMark  gives  Promecap  written  notice  of its
                                    intention to sell to Promecap the Convertible Debentures.

Maturity Date of
Debentures:                         3 years from date of issuance.

Interest Payment Dates:             The Debenture  bears interest at a rate of 12% per annum,  payable  annually in
                                    arrears in cash.

Conversion Feature:                 At any time,  the purchaser may convert the Debenture  into common stock at the
                                    Conversion Price in effect.

Conversion Price:                   75% of the average of the closing sales prices for the UNMG Common Stock for the
                                    thirty (30) consecutive trading days ending the trading day before UniMark
                                    gives Promecap written notice of its intention to sell to Promecap the
                                    Convertible Debenture.

Anti-dilution Protection:           Proportional  adjustments  for  splits,  dividends,  recapitalization, rights
                                    offerings and the like.

Optional Redemption:                At any time,  UniMark may redeem the Debenture,  at its option,  in whole or in part, with 20
                                    trading days notice, at the following redemption prices:

                                    (a)     Through  the first  anniversary  date of the  issuance,  at 113% of the
                                            principal amount of the Debenture outstanding.

                                    (b)     After the second anniversary date
                                            through the third anniversary date
                                            of the issuance, at 108% of the
                                            principal amount of the Debenture
                                            outstanding.

                                    Payment for redemption of the Debenture will
                                    include amounts due for accrued and unpaid
                                    interest, if any, to the date of redemption.

Ranking of Debenture:               The Debenture will be a general unsecured obligations, fully
                                    and completely subordinated in right and payment to all
                                    existing and future senior debt of UniMark, including all
                                    principal, interest, fees, expenses, penalties, overdrafts
                                    and swap obligations, under a subordination agreement
                                    satisfactory in form and content to the holder of the senior debt.
</TABLE>

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<TABLE>

<S>                                 <C>
Voting Rights:                      No voting rights until conversion.

Registration                        Rights: UniMark will grant Promecap one
                                    demand registration and unlimited number of
                                    customary piggyback registration rights with
                                    respect to shares of UNMG common stock
                                    underlying the Debenture.

Indemnification:                    Normal indemnification.
</TABLE><PAGE>   1
Exhibit 4.3(c)
LANVISION SYSTEMS, INC.

AMENDMENT TO LOAN AGREEMENT

                           AMENDMENT TO LOAN AGREEMENT

         THIS AMENDMENT TO LOAN AGREEMENT ("Amendment") is executed pursuant to
and is made a part of the Loan and Security Agreement dated July 17, 1998, by
and between LANVISION SYSTEMS, INC., a Delaware corporation ("Borrower"), and
THE HILLSTREET FUND, L.P., a Delaware limited partnership ("Lender"), as amended
by letter agreements dated March 18, 1999, April 12, 1999 and September 14, 1999
(as amended, the "Loan Agreement").

         WHEREAS, LanVision, Inc., an Ohio corporation and wholly-owned
subsidiary of Borrower ("Seller"), has entered into a certain Asset Purchase
Agreement with Smart Professional Photocopy Corporation, a California
corporation ("Smart"), dated as of January 20, 2000 (the "Purchase Agreement")
pursuant to which Seller shall sell to Smart a certain Data Center (as defined
in the Purchase Agreement) currently operated and maintained by Seller (the
"Sale Transaction");

         WHEREAS, Borrower desires to obtain the consent of Lender to the Sale
Transaction in accordance with the terms of the Purchase Agreement and the Loan
Agreement;

         WHEREAS, Borrower and Lender wish to further amend the Loan Agreement
in accordance with the terms and provisions hereof.

         NOW, THEREFORE, the parties agree as follows:

         Consent to Sale Transaction. Lender hereby consents to the Sale
Transaction in accordance with the terms of the Purchase Agreement.

         Waiver. Lender hereby agrees to waive Borrower's compliance with the
following covenant set forth in the letter agreement dated September 14, 1999:

"For the period August 1, 1999 through January 31, 2000, LanVision will execute
     new software license contracts, either directly or indirectly, through
     LanVision's distribution partners, with a net software value to LanVision
     of at least One Million Five Hundred Thousand and 00/100 Dollars
     ($1,500,000.00)."
<PAGE>   2

         Amendments to Loan Agreement. The following amendments shall be made to
the terms of the Loan Agreement:

         Definitions. Section 1.1 of the Loan Agreement shall be amended to add
the following definitions:

         "EBIT" for any period shall mean, without duplication (i) net income
         (or net loss) of the Borrower for such period determined in accordance
         with GAAP; plus (ii) for such period any Interest Expense deducted in
         the determination of net income; plus (iii) any income and franchise
         taxes paid in cash and included in the determination of net income.

         "Interest Expense" means, for any period, the total amount of all
         charges for the use of funds (whether characterized as interest, debt
         service or otherwise) payable during such period with respect to all
         Indebtedness for borrowed money of Borrower for such period.

         "Sale Transaction" means the transactions contemplated by that certain
         Asset Purchase Agreement between LanVision, Inc. and Smart Professional
         Photocopy Corporation dated as of January 20, 2000.

         Proceeds from Sale Transaction. A new Section 5.18 shall be added to
the Loan Agreement to read as follows:

         "Section 5.18 Proceeds from Sale Transaction. Borrower shall apply, or
         cause to be applied, the Net Proceeds received from the Sale
         Transaction, to increase the minimum cash balance and investment
         requirements contained in Section 6.7 of the Loan Agreement. For
         purposes of this Section 5.18, "Net Proceeds" shall mean the gross cash
         proceeds received from the Sale Transaction less the following
         transaction costs: (a) fees and costs in the approximate amount of
         Sixty-Three Thousand and 00/100 Dollars ($63,000.00) related to the
         Oracle software license, (b) the reasonable fees and costs of attorneys
         in the approximate amount of Sixty Thousand and 00/100 Dollars
         ($60,000.00), and (c) other fees and expenses, if any, separately
         agreed to by Lender, not to exceed the amount of Seventy-Five Thousand
         and 00/100 Dollars ($75,000.00)."

         Minimum Revenues and EBIT. Section 6.4 of the Loan Agreement shall be
amended in its entirety to read as follows:

         "Section 6.4 Minimum Revenues and EBIT.
<PAGE>   3
                  (a) Minimum Revenues. On each Computation Date set forth
         below, the Borrower shall not permit its total cumulative revenues
         (calculated for the period of time beginning on February 1, 2000
         through such Computation Date) to be less than the minimum amount set
         forth below:

<TABLE>
<CAPTION>
                                         MINIMUM
         COMPUTATION DATE          CUMULATIVE REVENUES
         ----------------          -------------------
<S>                                <C>
         April 30, 2000               $ 1,800,000.00
         July 31, 2000                $ 5,000,000.00
         October 31, 2000             $ 8,000,000.00
         January 31, 2001             $11,000,000.00
</TABLE>

                  (b) Minimum EBIT. On each Computation Date set forth below,
         the Borrower shall not permit its total cumulative EBIT (calculated for
         the period of time beginning on February 1, 2000 through such
         Computation Date) to be less than the minimum amount set forth below:

<TABLE>
<CAPTION>
                                       MINIMUM
         COMPUTATION DATE          CUMULATIVE EBIT
         ----------------          ---------------
<S>                                <C>
         April 30, 2000            ($1,000,000.00)
         July 31, 2000             ($  700,000.00)
         October 31, 2000          Break Even
         January 31, 2001           $  500,000.00
</TABLE>

         Borrower and Lender shall amend this Agreement on or before February
         28, 2001, to provide covenant compliance (at minimum levels acceptable
         to Lender) under Sections 6.4(a) and 6.4(b) above for April 30, 2001
         and each Computation Date thereafter."

         Net Worth. Section 6.5 of the Loan Agreement shall be amended in its
entirety to read as follows:

         "Section 6.5 Net Worth. At all times during the term of this Agreement,
         Borrower shall maintain a minimum Net Worth of Two Million and 00/100
         Dollars ($2,000,000.00)."

         Minimum Cash and Investments. Section 6.7 of the Loan Agreement shall
be amended in its entirety to read as follows:

         "Section 6.7 Minimum Cash and Investments. The Borrower shall at all
         times maintain on its balance sheet total cash and investments (as
         described in Section 6.10(b)) of at least Two Million Seven Hundred
         Thousand and 00/100 Dollars ($2,700,000.00); provided, that upon
         consummation of the Sale Transaction, Borrower shall at all times
<PAGE>   4
         thereafter maintain on its balance sheet total cash and investments (a
         described in Section 6.10(b)) of at least Four Million Four Hundred
         Thousand and 00/100 Dollars ($4,400,000.00) and as increased by any
         cash payments received by Borrower under that certain Promissory Note
         from Smart beginning with the first monthly installment due thereunder
         on March 1, 2000."

         Reaffirmation of Covenants, Representations and Warranties. Borrower
hereby agrees and covenants that all representations and warranties in the Loan
Agreement including, without limitation, all of those representations and
warranties set forth in Article 4, are true and accurate as of the date hereof.
Borrower further reaffirms all covenants in the Loan Agreement and reaffirms
each of the covenants set forth in Articles 5 and 6 thereof, as if fully set
forth herein, except to the extent modified by this Amendment.

       Remainder of page intentionally left blank. Signature page follows.
<PAGE>   5
         IN WITNESS WHEREOF, the parties hereto have executed this Amendment to
Loan Agreement as of the 11th. day of February, 2000.

LENDER:                                     BORROWER:

THE HILLSTREET FUND, L.P.                   LANVISION SYSTEMS, INC.

By:      HillStreet Capital, Inc.
Its:     Investment Manager                 By:   / s / J. Brian Patsy
                                               -----------------------------
                                               J. Brian Patsy, President and
                                               Chief Executive Officer
By:   / s / Chris Meininger
   -----------------------------------
   Christian L. Meininger, President

Date:                                       Date:  2/11/00

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