<PAGE>
                                                                   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.
<PAGE>

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.
<PAGE>

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.

<PAGE>

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
------------------------------------
Gregory J. Owens


Accepted by Raghavan Rajaji
/s/ Raghavan Rajaji
------------------------------------