Manual Exchange amount posting

Hi Experts...

Can u give some advice: I have an error during amount receipt for export sales.

I am trying to post transaction in F-28, While doing that T.Code. I am passing the transaction like

Bank Account Debit

Customer Account Credit

Becaz of foreign currency the difference amount is crediting/debiting in Gain or loss account. I need to post that difference manually.

I removed the Gain/Loss account in T.Code: FBKP in automatic postings. Still i am facing the problem. Is there any others changes to be done for doing that transacton.

Thanx in advace,

Regards,

Bhaskar