Sales Document Billing Conditions

HI All,

I have a peculiar requirement with the sales document billing conditions. the requirement is that when a delivery invoice is created with reference to a sales document, then my billing conditions value has to be changed depending upon the delivery invoice quantity and delivery invoice batch value. i have written a code to process the same with the help of a form routine (VOFM). but the problem is the user has to come back to VA02 screen again and press the update button to change the value. but the user wants the values to be changes automatically when the delivery invoice is created . can any when help me on this. any user exit or badi names can be used for this requirement ?

Thanks and Regards,

M.Phanindra