Modifier - Promotional Limits

Hi Pricing guru's

Scenario: Customer negotiated a program for 100 units @ $20 each

Customer sends data about consumption example 10 units

A credit memo is entered in OM for -10 units so that we can issue the credit to the customer.

And benefit should stop after consuming all 100 units.

Using Modifier - promotional limits functionality - units that we entered in Credit memo are adding up to the max ceiling that we setup i.e.. now the units available total is 100+10 i.e. 110 units

Any suggestions on how to achieve the above functionality.

Regards,

Srini .