Document:

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                                                                    EXHIBIT 10.5

                            STOCK PURCHASE AGREEMENT

      This Stock Purchase Agreement (the "Agreement"), dated as of September 28,
2001, is made by and between Mannatech, Incorporated, a Texas corporation (the
"Company") and Ray Robbins, an individual residing in Grand Prairie, Texas
("Purchaser").

      WHEREAS, the Company desires to sell Eight Hundred Fifteen Thousand
(815,000) shares of common stock, $0.0001 par value per share (the "Shares") to
Purchaser, and Purchaser desires to purchase the Shares from Company, subject to
the terms and conditions of this Agreement.

      NOW, THEREFORE, in consideration of the agreements and obligations set
forth herein and for other good and valuable consideration, the receipt and
sufficiency of which are hereby acknowledged, the parties hereto, intending to
be legally bound, hereby agree as follows:

                             STATEMENT OF AGREEMENT

      1. Purchase and Sale of the Shares. At the Closing (as hereinafter
defined) and upon the terms and subject to the conditions of this Agreement,
Company shall sell, transfer and deliver to Purchaser, and Purchaser shall
purchase from Company, the Shares. At the Closing, Company shall deliver to
Purchaser a certificate or certificates representing the Shares.

      2. Consideration and Payment for the Shares. As consideration for the
purchase of the Shares, Purchaser shall pay to Company the sum of Eight Hundred
Fifteen Thousand Dollars ($815,000.00) (the "Purchase Price"). The Purchase
Price shall be paid at the Closing by wire transfer of immediately available
funds.

      3. Closing. Subject to the conditions contained in this Agreement, the
deliveries contemplated by Sections 1 and 2 hereof (the "Closing") shall take
place at the offices of the Company simultaneously with the execution of this
Agreement.

      4. Representations and Warranties of Purchaser. Purchaser hereby
represents and warrants to Company as follows:

            (a) Authority. Purchaser has full legal capacity to enter into this
Agreement and to consummate the transactions contemplated hereby. This Agreement
has been duly executed and delivered by Purchaser and constitutes a valid and
binding obligation of Purchaser, enforceable in accordance with its terms,
except as enforceability may be limited by bankruptcy, insolvency or other
similar laws or equitable principles affecting the enforcement of creditors'
rights generally.

            (b) Conflict. The execution, delivery and performance of this
Agreement by Purchaser will not conflict with, result in any breach of or
constitute a default under any
<PAGE>

agreement, instrument, order, judgment, decree, law or governmental regulation
to which Purchaser is subject.

            (c) Investment Representations.

                  (i) Purchaser understands that the Shares have not been
registered under the Securities Act. Purchaser is acquiring the Shares for
investment purposes only and is not purchasing the Shares with a view to the
sale or distribution of any part thereof.

                  (ii) Purchaser has made such investigation into the Company
that Purchaser considers necessary and appropriate to its purchase of the
Shares, is capable of evaluating the merits and risks of its purchase of the
Shares, and is relying solely upon such investigation and not upon any
representation or warranty made by the Company, other than the representations
and warranties specifically made in this Agreement.

                  (iii) Purchaser qualifies as an "accredited investor" as such
term is defined in Regulation D promulgated under the Securities Act of 1933, as
amended.

      5. Miscellaneous.

            (a) Survival of Representations and Warranties. All representations,
warranties contained herein shall survive the execution and delivery of this
Agreement and the Closing, regardless of any investigation at any time made by
or on behalf of any party hereto.

            (b) Further Assurances. At the Closing and thereafter, from time to
time and without additional consideration, Company and Purchaser, as the case
may be, shall execute and deliver, or cause to be executed and delivered, such
additional or further transfers, assignments, endorsements, consents and other
instruments as Purchaser or Company, as the case may be, may reasonably request
for the purpose of effectively carrying out the transactions contemplated by
this Agreement.

            (c) Successors and Assigns. This Agreement will be binding upon,
inure to the benefit of, and be enforceable by the parties hereto and their
respective successors and assigns.

            (d) Amendments. This Agreement may not be amended, supplemented or
modified except by an instrument in writing signed by each of the parties
hereto.

            (e) Interpretation. The headings contained in this Agreement are for
reference purposes only and shall not affect in any way the meaning or
interpretation of this Agreement.

            (f) Counterparts. This Agreement may be executed in multiple
counterparts, each of which shall be deemed to be an original and which taken
together shall constitute one and the same agreement.

                                       2
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            (g) Entire Agreement. This Agreement (including the documents and
instruments referred to herein) (i) constitutes the entire agreement and
supersedes all prior agreements and understandings, both written and oral, among
the parties with respect to the subject matter hereof and (ii) is not intended
to confer upon any person other than the parties hereto any rights or remedies
hereunder.

            (h) GOVERNING LAW. THIS AGREEMENT SHALL BE GOVERNED BY AND CONSTRUED
IN ACCORDANCE WITH THE LAWS OF THE STATE OF TEXAS WITHOUT GIVING EFFECT TO THE
CONFLICTS OF LAWS RULES THEREOF. EACH PARTY HERETO AGREES THAT IT SHALL BRING
ANY ACTION OR PROCEEDING IN RESPECT OF ANY CLAIM ARISING OUT OF OR RELATED TO
THIS AGREEMENT OR THE TRANSACTIONS CONTAINED IN OR CONTEMPLATED HEREBY, WHETHER
IN TORT OR CONTRACT OR AT LAW OR IN EQUITY, EXCLUSIVELY IN THE STATE COURTS OF
TEXAS OR THE FEDERAL COURTS OF THE NORTHERN DISTRICT OF TEXAS (THE "CHOSEN
COURTS") AND (i) IRREVOCABLY SUBMITS TO THE EXCLUSIVE JURISDICTION OF THE CHOSEN
COURTS, (ii) WAIVES ANY OBJECTION TO LAYING VENUE IN ANY SUCH ACTION OR
PROCEEDING IN THE CHOSEN COURTS AND (iii) WAIVES ANY OBJECTION THAT THE CHOSEN
COURTS ARE AN INCONVENIENT FORUM OR DO NOT HAVE JURISDICTION OVER ANY PARTY
HERETO.

      IN WITNESS WHEREOF, the parties have executed this Agreement on the date
first above written.

                                        Mannatech, Incorporated

                                        By:  Stephen D. Fenstermacher
                                             -----------------------------------
                                        Its: Chief Financial Officer
                                             -----------------------------------

                                        PURCHASER:

                                        Marlin Ray Robbins
                                        ----------------------------------------
                                        Ray Robbins

                                       3<PAGE>

                                                                    EXHIBIT 10.1
              THIRD AMENDMENT TO FOURTH AMENDED AND RESTATED LOAN
              ---------------------------------------------------
                                   AGREEMENT
                                   ---------

     THIS THIRD AMENDMENT TO FOURTH AMENDED AND RESTATED LOAN AGREEMENT (this
"Amendment") is executed as of September 1, 2001, by and between AMX
 ---------
CORPORATION, a Texas corporation, formerly known as PANJA INC. ("Borrower") and
                                                                 ---------
BANK ONE, NA, successor by merger to BANK ONE, TEXAS, N.A., a national banking
association ("Lender").
              ------
                             W I T N E S S E T H:

     WHEREAS, Borrower and Lender entered into that certain Fourth Amended and
Restated Loan Agreement, dated September 1, 2000, pursuant to which Lender
agreed to make available to Borrower the Borrowing Base Line of Credit (as
therein defined) and the Advance/Term Facility (as therein defined)(as
heretofore or hereafter amended, the "Loan Agreement")(each capitalized term
                                      --------------
used herein, but not otherwise defined shall have the same meaning given to it
in the Loan Agreement); and

     WHEREAS, pursuant to the Loan Agreement, Borrower, Lender and Banc One
International Corporation executed that certain Exim Agreement (as defined in
the Loan Agreement) dated the date of the Loan Agreement, pursuant to which
Lender agreed to make available to Borrower a working capital loan for export
transactions in the amount of $4,000,000.00 (the "Exim Line of Credit") to be
                                                  -------------------
guaranteed by Exim Bank and included as a portion of the Borrowing Base Line of
Credit; and

     WHEREAS, in connection with the Loan Agreement, Borrower executed that
certain Renewal Promissory Note (the "Borrowing Base Note") dated the date of
                                      -------------------
the Loan Agreement in the stated principal amount of $10,000,000; and

     WHEREAS, in connection with the Exim Agreement, Borrower executed that
certain Promissory Note (the "Exim Note") dated the date of the Exim Agreement
                              ---------
in the stated principal amount of $4,000,000; and

     WHEREAS, Borrower and Lender entered into that certain Amendment to Fourth
Amended and Restated Loan Agreement (the "First Amendment") dated as of
                                          ---------------
January 5, 2001, pursuant to which Lender agreed to make available to Borrower
the 2001 Line of Credit (as therein defined); and

     WHEREAS, Borrower and Lender entered into that certain Second Amendment to
Fourth Amended and Restated Loan Agreement (the "Second Amendment") dated as of
                                                 ----------------
March 31, 2001, pursuant to which Lender agreed to modify certain covenants
contained in the Loan Agreement; and

     WHEREAS, Borrower has requested that Lender consent to an extension of the
Maturity Dates of the Borrowing Base Note and the Exim Note to September 14,
2001; and

     WHEREAS, subject to the terms and conditions herein contained, Lender is
willing to agree to such request.

     NOW, THEREFORE, in consideration of the covenants, conditions and
agreements hereinafter set forth, and for other good and valuable consideration,
the receipt and adequacy of which are all hereby acknowledged, Borrower and
Lender hereby covenant and agree as follows:

                                     - 1 -

Third Amendment to Fourth Amended and Restated Loan Agreement (AMX Corporation)
<PAGE>

                             ARTICLE I: AMENDMENTS
                             ---------------------

     Section 1.1. Extension of Maturity Dates. Subject to satisfaction of the
                  ---------------------------
conditions in this Amendment, the Maturity Dates set forth in the Borrowing Base
Note and the Exim Note shall be, and hereby are, renewed, modified and extended
to September 14, 2001, and in this regard, the Borrowing Base Note, the Exim
Note and all of the other Loan Documents are hereby renewed and modified (but
not extinguished) by extending the Maturity Dates to September 14, 2001.

                       ARTICLE II: CONDITIONS PRECEDENT
                       --------------------------------

     Section 2.1. Closing.  The closing (the "Closing") of the transactions
                  -------                     -------
contemplated by this Amendment shall occur on and as of the date that all
conditions hereto contained in Section 2.2 of this Amendment have been satisfied
                               -----------
(the "Amendment Closing Date").
      ----------------------

     Section 2.2. Conditions to the Closing. As a condition precedent to the
                  -------------------------
Closing, Borrower shall have executed and delivered to Lender this Amendment.

                          ARTICLE III: MISCELLANEOUS
                          --------------------------

     Section 3.1. Continuing Effect. Except as modified and amended hereby, the
                  -----------------
Loan Agreement and other Loan Documents are and shall remain in full force and
effect in accordance with their terms.

     Section 3.2. Payment of Expenses. Borrower agrees to pay to Lender the
                  -------------------
reasonable attorneys' fees and expenses of Lender's counsel and other expenses
incurred by Lender in connection with this Amendment.

     Section 3.3. Binding Loan Agreement. This Amendment shall be binding upon,
                  ----------------------
and shall inure to the benefit of, the parties' respective representatives,
successors and assigns.

     Section 3.4. Ratification. Except as otherwise expressly modified by this
                  ------------
Amendment, all terms and provisions of the Loan Agreement and the other Loan
Documents, shall remain unchanged and hereby are ratified and confirmed and
shall be and shall remain in full force and effect, enforceable in accordance
with their terms.

     Section 3.5. No Defenses. Borrower by its execution of this Amendment,
                  -----------
hereby declares that it has no set-offs, counterclaims, defenses or other causes
of action against Lender arising out of the Loans, the modification of the
Loans, any documents mentioned herein or otherwise; and, to the extent any such
setoffs, counterclaims, defenses or other causes of action may exist, whether
known or unknown, such items are hereby waived by Borrower.

     Section 3.6. Further Assurances. The parties hereto shall execute such
                  ------------------
other documents as may be necessary or as may be required, in the opinion of
counsel to Lender, to effect the transactions contemplated hereby and the liens
and/or security interests of all other collateral instruments, as modified by
this Amendment, including, without limitation, any modifications to any
financing statements. Borrower also agrees to provide to Lender such other
documents and instruments as Lender reasonably may request in connection with
the modification of the Loans effected hereby.

                                      -2-

Third Amendment to Fourth Amended and Restated Loan Agreement (AMX Corporation)
<PAGE>

     Section 3.7. Usury Savings Clause.  Notwithstanding anything to the
                  --------------------
contrary in this Amendment, the Notes or any other Loan Document, or in any
other agreement entered into in connection with the Notes or securing the
indebtedness evidenced by the Notes, whether now existing or hereafter arising
and whether written or oral, it is agreed that the aggregate of all interest and
other charges constituting interest, or adjudicated as constituting interest,
and contracted for, chargeable or receivable under the Notes or otherwise in
connection with the Notes shall under no circumstances exceed the maximum rate
of interest permitted by applicable law. In the event the maturity of the Notes
are accelerated by reason of an election by the holder thereof resulting from a
default thereunder or under any other document executed as security therefore or
in connection therewith, or by voluntary prepayment by the maker, or otherwise,
then earned interest may never include more than the maximum rate of interest
permitted by applicable law. If from any circumstance any holder of the Notes
shall ever receive interest or any other charges constituting interest, or
adjudicated as constituting interest, the amount, if any, which would exceed the
maximum rate of interest permitted by applicable law shall be applied to the
reduction of the principal amount owing on such Notes or on account of any other
principal indebtedness of the maker to the holders of such Notes, and not to the
payment of interest, or if such excessive interest exceeds the unpaid balance of
principal thereof and such other indebtedness, the amount of such excessive
interest that exceeds the unpaid balance of principal thereof and such other
indebtedness shall be refunded to the maker. All sums paid or agreed to be paid
to the holder of the Notes for the use, forbearance or detention of the
indebtedness of the maker to the holder of such Notes shall be amortized,
prorated, allocated and spread throughout the full term of such indebtedness
until payment in full for the purpose of determining the actual rate on such
indebtedness is uniform throughout the term thereof.

          The terms "maximum amount" or "maximum rate" as used in this Amendment
or the Notes, or in any other agreement entered into in connection with the
Notes or securing the indebtedness evidenced by the Notes, whether now existing
or hereafter arising and whether written or oral, include, as to Chapter 303 of
the Texas Finance Code (and as same may be incorporated by reference in other
statutes of the State of Texas), but otherwise without limitation, that rate
based upon the "weekly ceiling"; provided, however, that this designation shall
not preclude the rate of interest contracted for, charged or received in
connection with the Loans from being governed by, or construed in accordance
with, any other state or federal law, including but not limited to, Public Law
96-221.

     Section 3.8. Non-Waiver of Events of Default. Except as specifically
                  -------------------------------
provided herein, neither this Amendment nor any other document executed in
connection herewith constitutes or shall be deemed (a) a waiver of, or consent
by Lender to, any default or event of default which may exist or hereafter occur
under any of the Loan Documents, (b) a waiver by Lender of any of Borrower's
obligations under the Loan Documents, or (c) a waiver by Lender of any rights,
offsets, claims, or other causes of action that Lender may have against
Borrower.

     Section 3.9. Enforceability. In the event the enforceability or validity of
                  --------------
any portion of this Amendment, the Loan Agreement, the Notes, or any of the
other Loan Documents is challenged or questioned, such provision shall be
construed in accordance with, and shall be governed by, whichever applicable
federal or Texas law would uphold or would enforce such challenged or questioned
provision.

     Section 3.10. Counterparts. This Amendment may be executed in several
                   ------------
counterparts, all of which are identical, each of which shall be deemed an
original, and all of which counterparts together shall constitute one and the
same instrument, it being understood and agreed that the signature pages may be
detached from one or more of such counterparts and combined with the signature
pages from any other counterpart in order that one or more fully executed
originals may be assembled.

                                      -3-

Third Amendment to Fourth Amended and Restated Loan Agreement (AMX Corporation)
<PAGE>

     Section 3.11. Choice of Law.  THIS AMENDMENT SHALL BE GOVERNED BY, AND
                   -------------
CONSTRUED IN ACCORDANCE WITH, THE LAWS OF THE STATE OF TEXAS, EXCEPT TO THE
EXTENT FEDERAL LAWS PREEMPT THE LAWS OF THE STATE OF TEXAS.

     Section 3.12. Entire Loan Agreement. This Amendment, together with the
                   ---------------------
other Loan Documents, contain the entire agreements between the parties relating
to the subject matter hereof and thereof. This Amendment and the other Loan
Documents may be amended, revised, waived, discharged, released or terminated
only by a written instrument or instruments, executed by the party against which
enforcement of the amendment, revision, waiver, discharge, release or
termination is asserted. Any alleged amendment, revision, waiver, discharge,
release or termination which is not so documented shall not be effective as to
any party.

     THIS AMENDMENT AND THE OTHER LOAN DOCUMENTS REPRESENT THE FINAL AGREEMENT
     -------------------------------------------------------------------------
BETWEEN THE PARTIES RELATED TO THE SUBJECT MATTER THEREIN CONTAINED AND MAY NOT
-------------------------------------------------------------------------------
BE CONTRADICTED BY EVIDENCE OF PRIOR, CONTEMPORANEOUS OR SUBSEQUENT ORAL
------------------------------------------------------------------------
AGREEMENTS OF THE PARTIES. THERE ARE NO UNWRITTEN ORAL AGREEMENTS BETWEEN THE
-----------------------------------------------------------------------------
PARTIES.
-------

                                     - 4 -

Third Amendment to Fourth Amended and Restated Loan Agreement (AMX Corporation)
<PAGE>

     IN WITNESS WHEREOF, this Amendment is executed effective as of the date
first written above.

                         LENDER:
                         ------

                         BANK ONE, NA, successor by merger to BANK ONE, TEXAS,
                         N.A., a national banking association

                         By: /s/ Fred Points
                            ----------------------------------
                         Name: Fred Points
                              --------------------------------
                         Title: First Vice President
                               -------------------------------

                         BORROWER:
                         --------

                         AMX CORPORATION, a Texas corporation, formerly known
                         as PANJA INC.

                         By: /s/ Jean M. Nelson
                            ----------------------------------
                         Name: Jean M. Nelson
                              --------------------------------
                         Title: VP and Chief Financial Officer
                               -------------------------------

                                     - 5 -

Third Amendment to Fourth Amended and Restated Loan Agreement (AMX Corporation)

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