Condition Contract - Purchasing - Gross Value picked is Zero

Dear Gurus ,

We have a scenario where condition contracts are to be made against the vendors and we want the rebate value to be calculated basis of the gross value of the purchasing document .

Altough we have made a new field and assign BRWTR that is the standard field in table EKPO that holds the gross value .You can refer attachment for the assignnment but the business volume picked is bringing zero value through WB2R_busvol

You are guidance is required , whether this the correct procedure are follwoing to bring the gross value as the business volume or any alternate solution to it ?Are there abap exits to calculate business volume of condiiton contracts ?

Regards,

Ahmed