Discountable Amount on AP Invoice

Hi

We have a situation where we need to make the discountable amount greater than invoice amount because we add back few allowances that we get from Vendor but when we try to make the discountable amount more than invoice, its gives an error that it cannot exceed the invoice amount, is their a way around it?

Example

Invoice - 1000

Line 1 1100 (item cost)

Line 2 -100 (allowance)

Net Invoice = 1000

Discount terms 2% 5 NET 10

but the 2% is available on item cost  so we want to make discountable amount 1100 so we can get the 2% on it and have audit track.

Is it possible?

Thanks