Cash Discount (Returns)

The return will reference the existing billing document.

Yes ok, but if the invoice was made the discount because corresponded, the amount of the return must be on the invoice less the percentage, for example:
Invoice $ 100
Return $ 10
Total to pay $ 90

Return (full)
Copy control brings me with a value of $ 100. The return must be in $ 90 otherwise I am crediting $ 10 more to customer.
For this reason my question: how to know SAP if bill had the discount to move it to this amount?
thanks!

sorry
Invoice $ 100
Discount $ 10
Total to pay $ 90 (client)

Copy controls and pricing type!