Document:

<PAGE>   1
                                                                   EXHIBIT 10.36

[X]

                                AMENDMENT NO. 4

                 AMENDMENT NO. 4 dated as of December 8, 1999 between Nextel
Argentina S.R.L. (the "Borrower") and the parties below the title "Lenders" on
the signature pages hereof (the "Lenders").

                 The Borrower, the Subsidiary Guarantors named therein, the
Lenders and The Chase Manhattan Bank, as Administrative Agent, are parties to a
Credit Agreement dated as of February 27, 1998 (as amended by Amendment No. 1
and Waiver dated as of May 8, 1998, Amendment No. 2 dated as of September 30,
1998 and Amendment No. 3 dated as of May 12, 1999 and as further modified,
supplemented and in effect from time to time, the "Credit Agreement") and wish
to amend certain provisions of the Credit Agreement.

                 Accordingly, for good and valuable consideration, the receipt
and sufficiency of which are hereby acknowledged, the parties hereto hereby
agree as follows:

                 Section 1.  Definitions.  Except as otherwise defined in this
Amendment No. 4, terms defined in the Credit Agreement are used herein as
defined therein.

                 Section 2.  Amendments.  Subject to the satisfaction of the
conditions precedent specified in Section 4 hereof, the Lenders hereby agree
that the Credit Agreement is hereby amended as follows:

                 A.  References in the Credit Agreement (including references
to the Credit Agreement as amended hereby) to "this Agreement" (and indirect
references such as "hereunder", "hereby", "herein" and "hereof") shall be
deemed to be a reference to the Credit Agreement as amended hereby.

                 B.  Section 7.08(d) of the Credit Agreement is hereby amended
by deleting the existing Section 7.08(d) and replacing it with that set forth
below:

                 "(d)  Minimum Subscribers.  The Borrower will not permit the
         aggregate number of Subscribers to be less than the following
         respective numbers for the following respective periods:
<TABLE>
<CAPTION>
                               Period                  Minimum Number of Subscribers
                               ------                  -----------------------------
                <S>                                               <C>
                 From June 30, 1999                                35,000
                  through September 29, 1999

                 From September 30, 1999                           50,000
                  through December 30, 1999
</TABLE>
<PAGE>   2

                                      -2-

<TABLE>
                <S>                                              <C>
                 From December 31, 1999                            50,000
                  through March 30, 2000

                 From March 31, 2000                               60,000
                  through June 29, 2000

                 From June 30, 2000                                98,500
                  through September 29, 2000

                 From September 30, 2000                          112,500
                  through December 30, 2000

                 From December 31, 2000                           127,500
                  through March 30, 2001

                 From March 31, 2001                              145,000
                  through June 29, 2001

                 From June 30, 2001                               163,000
                  through September 29, 2001

                 From September 30, 2001                          182,000
                  through December 30, 2001

                 From December 31, 2001                           200,000
                  and at all times thereafter
</TABLE>

                 C.  Section 7.08(e) of the Credit Agreement is hereby amended
by deleting the existing Section 7.08(e) and replacing it with that set forth
below:

                 "(e)  Minimum Revenues.  The Borrower will not permit the
         aggregate amount of the revenues of the Borrower and its Subsidiaries
         from the operation of its network system for any period of four
         consecutive quarters ending during the following respective periods to
         be less than the following respective amounts:
<TABLE>
<CAPTION>
                               Period                              Amount
                               ------                              ------
                <S>                                         <C>
                 From June 30, 1999                          U.S. $25,000,000
                   through September 29, 1999

                 From September 30, 1999                     U.S. $33,500,000
                  through December 30, 1999

                 From December 31, 1999                      U.S. $35,000,000
                  through March 30, 2000

                 From March 31, 2000                         U.S. $40,000,000
                  through June 29, 2000

                 From June 30, 2000                          U.S. $65,500,000
                  through September 29, 2000

                 From September 30, 2000                     U.S. $76,000,000
                  through December 30, 2000

                 From December 31, 2000                      U.S. $88,000,000
                  through March 30, 2001

                 From March 31, 2001                        U.S. $107,000,000
                  through June 29, 2001

                 From June 30, 2001                         U.S. $123,500,000
                  through September 29, 2001
</TABLE>
<PAGE>   3
                                     -3-

<TABLE>
                 <S>                                        <C>
                 From September 30, 2001                    U.S. $141,000,000
                  through December 30, 2001

                 From December 31, 2001                     U.S. $160,000,000
                  and at all times thereafter
</TABLE>

                 Section 3.  Limited Waiver.  Subject to the satisfaction of
the conditions precedent specified in Section 4 hereof, the Lenders hereby
waive the requirement set forth in Section 6.01(f) of the Credit Agreement to
deliver financial projections for the fiscal year ending December 31, 2000, so
long as the Borrower delivers such projections on or before May 15, 2000.

                 Section 4.  Conditions Precedent.  The amendments to the
Credit Agreement set forth in Section 2 hereof, and the limited waiver of
Section 6.01(f) of the Credit Agreement set forth in Section 3 hereof, shall
become effective upon the date on which this Amendment No. 4 shall have been
duly executed and delivered by each of the Borrower and Lenders constituting
the Required Lenders, and the Consent and Agreement to Amendment set forth on
the signature pages below by each of the Relevant Parties shall have been duly
executed and delivered by each of the Relevant Parties.

                 Section 5.  Miscellaneous.  Except as herein provided, the
Credit Agreement shall remain unchanged and in full force and effect.  This
Amendment No. 4 may be executed in counterparts which, taken together, shall
constitute a single document and any of the parties hereto may execute this
Amendment No. 4 by signing any such counterpart.  Terms defined in the Credit
Agreement are used herein as defined therein.  This Amendment No. 4 shall be
governed by and construed in accordance with the law of the State of New York.

                 IN WITNESS WHEREOF, the parties hereto have caused this
Amendment No. 4 to be duly executed as of the date and year first above
written.

                               NEXTEL ARGENTINA S.R.L.

                               By: /s/ Ricardo Israele
                                   Title: Director de
                                          Administracion y Finanzas

                               Lenders
                               -------

                               THE CHASE MANHATTAN BANK

                               By: /s/ Tracey Navin Ewing
                               Title: Vice President

<PAGE>   4
                                     -4-

                               ING BANK N.V., CURACAO BRANCH

                               By: /s/ Augusti Faus Rodes
                                  Title: Attorney-in-Fact

                               By: /s/ Semeire Camargo Victor
                                  Title: Attorney-in-Fact

                               CREDIT SUISSE FIRST BOSTON

                               By: /s/ authorized signatory
                                  Title: Vice President

                               By: /s/ authorized signatory
                                  Title:  Managing Director

                               SOCIETE GENERALE

                               By: /s/ Javier Rocio
                                  Title: Director

                               VAN KAMPEN PRIME RATE INCOME TRUST (formerly
                                 known as Van Kampen American Capital
                                 Prime Rate Income Trust)

                               By: /s/ Darvin D. Pierce
                                  Title: Vice President

                               KZH IV LLC

                               By: /s/ Peter Chin
                                  Title:  Authorized Agent

                               MOTOROLA CREDIT CORPORATION

                               By: /s/ Gary Tatje
                                  Title:   Vice President

<PAGE>   5
                                     -6-

                       CONSENT AND AGREEMENT TO AMENDMENT

                 Each of the undersigned hereby (1) consents to the amendments
provided for in this Amendment No. 4, (2) agrees that each reference to the
Credit Agreement in each Loan Document (as defined in the Credit Agreement) to
which it is a party shall be a reference to the Credit Agreement as amended by
this Amendment No. 4 and (3) confirms its obligations under each Loan Document
to which it is a party after giving effect to the amendments set forth in this
Amendment No. 4.

NEXTEL INTERNATIONAL, INC.

By:  /s/ Michel Buhler
     Title: VP - Treasurer

NEXTEL INTERNATIONAL (ARGENTINA) LTD.

By: /s/ Mariana Vazquez
    Title:  Attorney-in-Fact

NEXTEL INTERNATIONAL (HOLDINGS) LTD.

By:  /s/ Mariana Vazquez
     Title: Attorney-in-Fact<PAGE>   1
                                                                   EXHIBIT 10.37
                       SUBSCRIBER UNIT PURCHASE AGREEMENT
                FOR COMMUNICACIONES NEXTEL DE MEXICO S.A. DE C.V.

            This Agreement is entered into between Motorola, Inc., by and
through its iDEN Subscriber Group, having a place of business at 8000 West
Sunrise Boulevard; Ft. Lauderdale, Florida 33322 (hereinafter "Seller" or
"MOTOROLA") and Communicaciones Nextel de Mexico S.A. de C.V., a company formed
under the laws of Mexico, with a place of business at Av. Revolucion #300,
Colonia Tacubaya Mexico, D.F. 11870 (hereinafter, together with any wholly-owned
subsidiary previously accepted by the Seller, collectively known as "Buyer").

            This agreement applies only for sales of Motorola iDEN subscriber
products for distribution outside of the United States. Buyer agrees to purchase
and Seller agrees to sell selected Motorola iDEN subscriber products
manufactured in Seller's facilities in Plantation, Florida, U.S.A., (hereinafter
"Products") during a term beginning on the date of the last signature hereto
("Effective Date"), and ending April 30, 2000 ("Initial Term"), under the terms
and conditions set forth in this Agreement. Buyer may purchase products
manufactured at Seller's other facilities under terms and conditions acceptable
to both parties at the time such purchases are desired.

PURCHASE ORDERS. All orders by Buyer shall be only upon the terms and conditions
of this Agreement. The only effect of any terms and conditions in Buyer's orders
or elsewhere shall be to request the time and number of units to be delivered,
subject to Seller's acceptance, but they shall not change, alter or add to the
terms and conditions of this Agreement in any other way. Except as provided
below, Seller's invoice shall also not change the terms and conditions of this
Agreement. Cancellation charges which shall include all costs incurred or
committed for may apply for any cancelled order. Seller shall use its reasonable
best efforts, including shipment to other Nextel properties, to mitigate any
costs incurred as a result of the Buyer's cancellation of an order within the
non-cancelable period. Purchase orders are non-cancelable fifteen (15) days or
less before requested ship date.

PRICING. Contract prices for Products require firm and prosecutable orders
thirty (30) days prior to requested ship date. Prices applicable to Products
sold pursuant to this Agreement ("Prices") are those negotiated from time to
time by Nextel Communications, Inc. and Motorola, Inc. In order for Prices to be
applicable, the purchase orders submitted to Seller must reference this
Agreement.

PAYMENT TERMS AND CONDITIONS. Seller will invoice Buyer for the Products on or
about the date of shipment thereof. Payment of each invoiced amount is due
within 30 days of the date of the invoice;[*]. The Buyer may prepay any invoice,
in part or in full, without penalty or premium at any time.

                                       1

     * Confidential portions omitted and filed separately with the Commission
pursuant to an application for confidential treatment pursuant to Rule 24b-2
under the Securities and Exchange Act of 1934, as amended.
<PAGE>   2
[*]

[*]

FORECASTS. During the term of this Agreement, Buyer shall provide Seller,[*] a
continuous usage forecast for [*] to assist Seller in maintaining an orderly
production flow for the purpose of Buyer's delivery requirements. Buyer shall
state in the [*] Forecast the Product model numbers and projected purchase
volume by units for [*]. The [*] Forecast does not constitute a commitment by
the Buyer; however, Buyer's failure to provide such information may be
considered cause by Seller for excusable delivery delay.

DELIVERY AND TITLE. All deliveries are FCA Seller's plant, Plantation, Florida
USA. Each such delivery will be separately invoiced. DELIVERY DATES ARE BEST
ESTIMATES ONLY. Title to the Products will pass to Buyer at the Seller's plant,
Plantation, Florida.

FORCE MAJEURE. Neither party shall be liable for any delay or failure to perform
due to any cause beyond its reasonable control. Causes include, but are not
limited to strikes, acts of God, acts of the other party, interruptions of
transportation or inability to obtain necessary labor, materials or facilities,
or default of any supplier, or delays in regulatory authorization or license
grant. The delivery schedule shall be considered extended by a period of time
equal to the time lost because of any excusable delay. In the event either party
is unable to wholly or partially perform for a period greater than forty-five
(45) days because of any cause beyond its reasonable control, either party may
terminate any delayed order without any liability.

LICENSE DISCLAIMER. Nothing contained herein shall be deemed to grant either
directly or by implication, estoppel, or otherwise, any license under any
patents, copyrights, trademarks or trade secrets of SELLER.

TAXES. Except for the amount, if any, of specific taxes stated in the Agreement,
Prices are exclusive of any amount for Federal, State and/or Local excise,
sales, use, property, retailer's, occupation or any other assessment in the
nature of taxes however designated, on the Products and/or services provided
under this Agreement. If any such excluded tax, exclusive however, of any taxes
measured by Seller's net income or taxes based on Seller's gross receipts or
based on Seller's franchise, is determined to be applicable to this transaction
or to the extent Seller is required to pay or bear the burden thereof, one
hundred percent (100%) thereof shall be added to the Prices and paid by Buyer.
Personal property taxes assessed on the Products after the date title transfers
under this paragraph, shall be the responsibility of Buyer. In the event Buyer
claims exemption from sales, use or other such taxes under this Agreement, Buyer
shall hold Seller harmless of any subsequent assessments levied by a proper
taxing authority for such taxes, including interest, penalties, and late
charges.

Upon Seller's request, Buyer shall produce sufficient evidence within thirty
(30) days of such request to prove that Buyer has fulfilled its obligation
relating to all taxes, duties, and fees. If any such taxes, duties, or fees are
determined to be applicable to this transaction and notwithstanding Buyer's
responsibility, Seller is required to pay or bear the burden thereof, then
Prices shall be increased by the amount of such taxes and any interest or
penalty, and Buyer shall pay to Seller the full amount of any such increase no
later than thirty (30) days after receipt of an invoice.

                                        2

     * Confidential portions omitted and filed separately with the Commission
pursuant to an application for confidential treatment pursuant to Rule 24b-2
under the Securities and Exchange Act of 1934, as amended.

<PAGE>   3

Customs clearance, or related costs pertaining to import of the Products are the
responsibility of Buyer. No such costs have been included in Seller's Prices.

TECHNICAL ASSISTANCE. Seller's warranty shall not be enlarged, and no obligation
or liability shall arise out of Seller's rendering of technical advice,
facilities or service in connection with Buyer's purchase of the Products
furnished.

LIMITATION OF LIABILITY. SELLER'S TOTAL LIABILITY, WHETHER FOR BREACH OF
CONTRACT, WARRANTY, NEGLIGENCE, STRICT LIABILITY IN TORT OR OTHERWISE, IS
LIMITED TO THE PRICE OF THE PARTICULAR PRODUCTS SOLD HEREUNDER WITH RESPECT TO
WHICH LOSSES OR DAMAGES ARE CLAIMED. BUYER'S SOLE REMEDY IS TO REQUEST SELLER AT
SELLER'S OPTION TO EITHER REFUND THE PURCHASE PRICE, REPAIR OR REPLACE PRODUCT
(S) THAT ARE NOT AS WARRANTED. IN NO EVENT WILL SELLER BE LIABLE FOR ANY LOSS OF
USE, LOSS OF TIME, INCONVENIENCE, COMMERCIAL LOSS, LOST PROFITS OR SAVINGS OR
OTHER INCIDENTAL OR CONSEQUENTIAL DAMAGES TO THE FULL EXTENT SUCH MAY BE
DISCLAIMED BY LAW.

LOGOS AND TRADEMARKS. In order that Seller may protect its trademarks, trade
names, corporate slogans, corporate logo, goodwill and product designations,
Buyer, without the express written consent of Seller, shall have no right to use
any such marks, names, slogans or designations of Seller in the sales, lease or
advertising of any products or on any product container, component part,
business forms, sales, advertising and promotional materials or other business
supplies or material, whether in writing, orally or otherwise. Buyer agrees to
provide Motorola with advertising material containing any Motorola marks for
approval before such marks are used. Motorola agrees to review Buyer's
advertising material in a reasonable time and to not unreasonably withhold
approval thereof. To minimize the impact of this review on the Buyer, Motorola
will provide Buyer with guidelines on the use of Motorola logos. Compliance with
these guidelines shall serve to relieve Buyer from having to submit
advertisements for prior approval.

PARTY RELATIONSHIP. This Agreement does not create any agency, joint venture or
partnership between Buyer and Seller. Buyer shall not impose or create any
obligation or responsibility, express or implied, or make any promises,
representations or warranties on behalf of Seller, other than as expressly
provided herein.

WAIVER. The failure of either party to insist in any one or more instances, upon
the performance of any of the terms or conditions herein or to exercise any
right hereunder shall not be construed as a waiver or relinquishment of the
future performance of any such terms or conditions or the future exercise of
such right but the obligation of the other party with respect to such future
performance shall continue in full force and effect.

DEFAULT. In the event that either party shall be in breach or default of any of
the terms or conditions of this Agreement and such breach or default shall
continue for a period of thirty (30) days after the giving of written notice by
the non-defaulting party, then subject to the other terms and conditions of this
Agreement, the non-defaulting party, in addition to other rights and remedies it
may have in law or equity, shall have the right to immediately cancel this
Agreement without any charge or liability whatsoever.

                                        3

<PAGE>   4

DISPUTE RESOLUTION. The parties agree that any claims or disputes will be
submitted to non-binding mediation prior to initiation of any formal legal
process. Costs of mediation will be shared equally.

WARRANTY. The warranty document supplied with the Product at the time of
shipping shall govern the warranty coverage provided under this Agreement.

GENERAL. This Agreement constitutes the entire and final expression of agreement
between the parties pertaining to the subject matter hereof and supersedes all
other communications, oral or written, between the parties. No alterations or
modifications of this Agreement shall be binding upon either Buyer or Seller
unless made in writing and signed by an authorized representative of each. If
any term or condition of this Agreement shall to any extent be held by a court
or other tribunal to be invalid, void or unenforceable, then that term or
condition shall be inoperative and void insofar as it is in conflict with law,
but the remaining rights and obligations of the parties shall be construed and
enforced as if this Agreement did not contain the particular term or condition
held to be invalid, void or unenforceable. Buyer shall make no assignment of
this Agreement or of any right granted herewith without the prior written
consent of SELLER. Any assignment of this Agreement is null and void. The laws
of the State of Illinois of the United States shall govern this Agreement.

            Unless this Agreement is superseded by a new agreement, or otherwise
terminated pursuant to the terms contained herein, this Agreement will continue
in effect beyond its Initial Term until terminated by either party upon thirty
(30) days prior written notice.

            IN WITNESS WHEREOF, the parties have caused this Agreement to be
 executed by their duly authorized representatives effective as of the 23
 day of July, 1999.

       SELLER:                                       BUYER:

       MOTOROLA, INC.                                COMMUNICACIONES NEXTEL DE
                                                     MEXICO S.A. DE C.V.

By:    /s/ Mark Shockley                       By:   /s/ Alejandro A. Miranda
       ---------------------------                   --------------------------
       Signature                                     Signature

Printed Name: MARK SHOCKLEY                   Printed Name: Alejandro A. Miranda
                                                           ---------------------

Title: Vice President & General Manager       Title: Treasury Manager
       iDEN International Markets Division          ---------------------------
       iDEN Subscriber Group

Date: July 23, 1999                         Date: July 23, 1999
     --------------                              --------------

                                        4

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