Hi Vinod,
The system is behaving correctly while posting Gain/Loss as the differential of the exchange rates on the pre-mature settlement date.
Nevertheless if you wish to change the exchange rate being picked up, please follow the steps below on the original contract and offsetting contract:-
1. Go to FTR_EDIT and click on Change for this Transaction.
2. Go the the Cash Flow Tab and Right click on the USD Flow and select Edit Flow.
3. Under Local Currency, check mark exchange rate fixed and input your desired exchange rate.
4. Click on Check and save the transaction.
5. Do a TPM27 to update and then try posting after releasing.
Please let me know if this sufficies your requirement.
Regards,
Rohit