Sales Invoice Return for Invoices with Additional Discount Amount ( Not Percentage )

Hi,

When we do partial sales return of a Sales Invoice which has Additional Discount added by just putting the amount and not %, whats the best way to manage this. Is there any issue with back calculation of Additional Discount % when entering Additional Discount amount so that when we do partial return, discount also gets adjusted according to the percentages like it currently does.

Thanks,
Palash