Planned costs with Subcontracting Orders

Dear MM Gurus.

Can anybody tell me how can I make the Planned Costs in Subcontracting Orders to be assumed by the incoming material instead of a G/L account determined by the FRN operation in OBYC transaction?

When I try to use a inventory account in FRN operation, SAP tells me that the account is already been used in BSX operation.

Thanks in advance.

Miguel