Rebate agreement: solution to reverse a credit memo for partial payment and accrual

Hi Guru's.

I try to explain my need with an example.

My process to settle partial rebate agreement consist in to creation of 3 credit memos: at the first, we generate an accrual with document type B4; when we've to pay the customer we create a payment that produces 2 documents: a document B4 that reverses the first accrual and a document type B3 to the customer.

For example, we have done an accrual for 100€ (credit memo with document type B4); then we create a payment for 100€ that produces a credit memo B4 to reverse the first accrual and a credit memo B3 to the customer.

Now, assuming we've made a mistake: we had to pay only 80€.

So how can reverse 20€ using the same process in order to re-estabilish the values for payment and accrual?

Consider that i cannot enter manually any pricing condition cause the documents are created by an automatic process.

Thank you in advance!

BR

Maurizio