Billing Document - Pricing Procedure.

We want that at the time of billing Document,system should accept one extra Discount condition at Item Level automatically with some value which we want to put in the saystem or we will control using Exit & it should come on Calucating total Value.

I want to know that can we do this.I dnt want to put DIscount Value at the time of Sales Order creation,but in billing doucment it should show one extra condition .

Can it be done.

Please help me to build this process.

Viru.