modify discount in selected invoices - PPR

 in 11i after invoices selected for payment i can change discount then [tab] and the proposed payment amount is changed to reflect discount. where can i do this in r12 PPR.. In scheduled selected invoices  i want to change discount and have payment amount changed after tabbing over field. even if i hit calculate it still doesn't work. how do you modify the discount in r12 selected inovces? do i have to manually calculcate change to payment amount and enter that amount in payment amount field? what actually equates to the proposed amount in r12.