From SD side, you need to create a new condition type with calculation type as Percent and assign it in your pricing procedure. Ask your FI guys to create a new Account Key and a G/L Account to post the retension values. Once this this done, assign the Account Key against the new condition type in your pricing procedure followed by assigning the G/L Account in VKOA. Also you need to maintain condition record to the extent of what percent you want to post in the special G/L Account. Once all these are done, create a billing document and test how values are fetching.
G. Lakshmipathi