Varun,
What is your price Control is it S or V?
The exchange rate differences can be captured in MIRO (invoice Booking ) as well as at the Payment transaction to the vendor.
If you really need to load the gain/loss account for the exchange rate difference you can configure in OB08, the gain /loss account.
Exchange rate differences in the case of open items arise when an invoice relating to a purchase order is posted with a different exchange rate to that of the goods receipt and the material cannot be debited or credited due to standard price control or stock under coverage/shortage.
So this can be used only if we have price control S.
In case of MAP the price has to go to Inventory account PPV.
If you require to post exchange rate difference into a Gain / Loss account, instead of capturing the exchange rate difference at the Invoice booking you can capture the same at Payment.
In OB09, you can configure the Gain / Loss account for exchange rate differences based on the Reconciliation account.
So there is choices available for that.
I hope it helps
Best regards
Hrusikesh