Discount Taken P0413M and Discount Available P0411

Hello,

I have two questions regarding accounts payable processes.

1. I am trying to enter a Discount to one voucher during the manual payment process (P0413M), but if I don’t have “Discount Available (ADSC)” field with value on F0411 for the same voucher it doesn’t let me enter the discount on “Discount Taken (ADSA)” field. See error on the document attached.

       My question is, is it possible to add Discount Take on P0413M without having the Discount Available? I don’t want to change the voucher every time I negotiate something different with my           supplier.

2.When I try to access the voucher on P0411 to add the Discount Available, even though the period is open for that company (00256) it gives me an error of “Prior End Balance Forward Invalid”.