Hi
If I understand your question correctly, you would not like to post Forex realised gain/loss whenever bank to bank transfer posting are uploaded by bank statement processing, correct?
I could think of at least 2 possible approaches.
1. The approach which enlist the standard configuration only: You could use a separate open item clearing account only for this bank to bank transfer (both Payment and Bank statement) and by OBA1 configuration, you point the realised gain/loss account determination to Balance sheet (i.e. Clearing account adjustment account), so though the system will insert an additional line based on the difference of Exchange rate, the net effect is correct.
2. Alternatively, implement User exit or BTE (probably event 1120) to manipulate FB05 posting only in the case where bank to bank transfer document is concerned, to use the same Exchange rate as the original document that is being cleared.
Hope this helps
Cheers
Taro