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                                                                   EXHIBIT 10.39

                          [LETTERHEAD OF MANUGISTICS]

December 6, 1999

Mr. Raghavan Rajaji
2612 Regatta Drive
Plano, Texas 75093

Dear Raj,

On behalf of Manugistics, Inc., as well as its parent company, Manugistics
Group, Inc., and the Board of Directors of both, we are very pleased to confirm
our verbal offer for the position of Executive Vice President and Chief
Financial Officer as of December 6, 1999. This position reports to the President
and CEO of Manugistics.

Cash Compensation. In this position, your annual base salary is $275,000. In
addition, you are eligible to receive an annual incentive bonus of up to
$180,000. The incentive bonus is payable as follows: 50% based on the financial
performance of the Company and 50% based upon management objectives. This will
be paid on a quarterly basis. The bonuses will be payable under the plan
submitted by you within the first ninety (90) days of your employment and
approved by the President and CEO.

Stock Options. We are also pleased to offer you a stock option package as
follows:

An option, granted as of your date of hire, to purchase 225,000 shares of
Manugistics Group, Inc. common stock ("Manugistics Stock") in accordance with
the Manugistics 1998 Stock Option Plan (the fair market value on your date of
hire is determined by taking the average of the high and low stock price for the
previous business day).

The vesting period for the stock options under this first option shall commence
on the date of grant and vest over a four (4) year period, in equal monthly
increments.
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Mr. Raghavan Rajaji
December 6, 1999
Page Two

Additional Stock Award. In addition, you will receive an option to purchase
additional shares of Manugistics Stock as follows:

 .    If Manugistics Stock price reaches $35/share or a $200M annualized run rate
     defined by a $50M revenue quarter, which ever comes first, you will have
     the option to purchase 35,000 shares on the last day of the fifteen (15)
     day period during which the stock maintains a closing price of at least
     $35/share.

In keeping with Manugistics policy, all offers are contingent upon successful
completion of employment references.

Benefits.  Effective on your first day of employment and as a key executive in
the Company, you will be eligible for the comprehensive Manugistics benefits
program in accordance with the Company's written plans and which includes:

     .    Stock Options - as set forth above
     .    Employee Stock Purchase Plan
     .    401(k) Retirement Plan
     .    Comprehensive Medical Care
     .    Dental Care
     .    Employee Vision Care
     .    Life Insurance
     .    Accidental Death and Dismemberment Insurance
     .    Long Term Disability
     .    Vacation (please refer to the attached benefits sheet)

Relocation.  You are also eligible to receive relocation benefits. Please see
the attached Manugistics Relocation Expense Reimbursement Agreement and
Relocation Policy. The details of your relocation package will be agreed upon by
you and the President and CEO.
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Mr. Raghavan Rajaji
December 6, 1999
Page Three

Termination. If the Company terminates your employment for reasons other than
cause, you will receive your base salary and benefits in accordance with the
Company's payroll practices during the six (6) month period commencing on your
termination date ("severance period"). The foregoing salary and benefits
payments will cease if you secure alternative employment during the severance
period. In addition, if the Company terminates your employment for reasons other
than cause, the Company will continue the monthly vesting of the options granted
to you pursuant to this letter for a period of six (6) months following your
termination date.

Change of Control. In the event that the Company has a change of control, which
is defined as fifty one percent (51%) of the Company's voting stock having a
change in ownership: (a) if your responsibilities are not affected, fifty
percent (50%) of your outstanding options set out above shall immediately vest;
(b) if your responsibilities are significantly diminished or you are
constructively terminated, i.e., your responsibilities no longer consist of
those reasonably associated with the position of Executive Vice President and
Chief Financial Officer, one hundred percent (100%) of the outstanding options
set out above shall immediately vest.

Final Determination by Board. All compensation and benefits included as part of
this offer will conform to the Company's standard policies, practices and plans.
In the event of any question with regard to the compensation and benefits
described in this letter, the Compensation Committee of the Board of Directors
will make the final determination with regard to any interpretation relating to
the elements of your compensation package.

In keeping with Manugistics policy, all offers are contingent upon execution of
Manugistics, Inc.'s Conditions of Employment.

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Mr. Raghavan Rajaji
December 6, 1999
Page Four

Please signify your acceptance of this offer by signing and dating this letter.

We look forward to your joining Manugistics as Executive Vice President and
Chief Financial Officer and we are confident that the association will be
mutually rewarding.

Sincerely,

/s/ Gregory J. Owens
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Gregory J. Owens

Accepted by Raghavan Rajaji
/s/ Raghavan Rajaji
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                                 Exhibit 10.28

                                PROMISSORY NOTE

$21,461.54                                             November 30, 1999
                                                            Dallas, Texas

     FOR VALUE RECEIVED, James P. Farley, ("Borrower"), hereby unconditionally
promises to pay to the order of RF Monolithics, Inc., a Delaware corporation
("Lender"), in lawful money of the United States of America and in immediately
available funds, the principal sum of Twenty-one Thousand, Four Hundred
Sixty-one Dollars and Fifty-four Cents ($21,461.54) (the "Loan") together with
accrued and unpaid interest thereon, payable on the dates and in the manner set
forth below.

     1.   Principal Repayment. The outstanding principal amount of the Loan
shall be due and payable on November 30, 2002.

     2.   Interest Rate. Borrower further promises to pay interest on the
outstanding principal amount hereof from the date hereof until payment in full,
which interest shall be payable at the rate of Four and Ninety/One Hundredths
percent (4.90%) per annum or the maximum rate permissible by law (which under
the laws of the State of Texas shall be deemed to be the laws relating to
permissible rates of interest on commercial loans), whichever is less. Interest
shall be payable quarterly in arrears not later than the first day of each
calendar quarter for the preceding quarter and shall be calculated on the basis
of a 360-day year for the actual number of days elapsed.

     3.   Place of Payment. All amounts payable hereunder shall be payable at
the office of Lender, unless another place of payment shall be specified in
writing by Lender.

     4.   Application of Payments. Payment on this Note shall be applied first
to accrued interest, and thereafter to the outstanding principal balance
thereof.

     5.   Secured Note. The full amount of this Note is secured by the
Collateral identified and described below. As collateral security for prompt and
complete payment and performance of all obligations of Borrower under this Note
and to induce lender to extend credit, Borrower hereby assigns, conveys, grants,
pledges, and transfers to and creates in favor of Lender a security interest in
all goods and personal property of Borrower whether tangible or intangible and
whether now or hereafter owned by Borrower ("Collateral").

     6.   Default. Each of the following events shall be an "Event of Default"
hereunder:

          (a)  Borrower fails to pay timely any of the principal amount due
under this Note on the date the same becomes due and payable or any accrued
interest or other amounts due under this Note on the date the same becomes due
and payable or within five (5) calendar days thereafter;

          (b)  Borrower files any petition or action for relief under any
bankruptcy, reorganization, insolvency, or moratorium law, or any other law for
the relief of, or relating to, debtors, now or hereafter in effect, or makes any
assignment for the benefit of creditors, or takes any corporate action in
furtherance of any of the foregoing; or

          (c)  An involuntary petition is filed against Borrower (unless such
petition is dismissed or discharged within sixty (60) days), under any
bankruptcy statute now or hereafter in effect, or a custodian,
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receiver, trustee, assignee for the benefit of creditors (or other similar
official) is appointed to take possession, custody, or control of any property
of Borrower.

Upon the occurrence of an Event of Default hereunder, all unpaid principal,
accrued interest and other amounts owing hereunder shall, at the option of
Lender, be immediately collectible by Lender pursuant to applicable law.

     7.   Waiver. Borrower waives presentment and demand for payment, notice of
dishonor, protest and notice of protest of this Note, and shall pay all costs of
collection when incurred, including, without limitation, reasonable attorneys'
fees, costs, and other expenses.

     The right to plead any and all statutes of limitations as a defense to any
demands hereunder is hereby waived to the full extent permitted by law.

     8.   Governing Law. This Note shall be governed by, and construed and
enforced in accordance with, the laws of the State of Texas, excluding conflict
of laws principles that would cause the application of laws of any other
jurisdiction.

     9.   Successors and Assigns. The provisions of this Note shall inure to the
benefit of and be binding on any successor to Borrower and shall extend to any
holder hereof.

Borrower                                RF Monolithics, Inc.

/s/ James P. Farley                     /s/ David M. Kirk
----------------------                  ---------------------------------
James P. Farley                         David M. Kirk
                                        President and CEO

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