Hi
The reversal of FX Un realized gain/Loss depends on the accounting principles in your country. Generally, in most markets, it is a accounting principle to convert all your open FX Transactions/balances to local currency for balance sheet purpose, which is supposed to be in local currency and than reverse the same on the first day of the next period. This is done so that you can book actual FX Gain/loss as and when it happens. So it depends on how you want to account such transaction. I believe, the issue is more of a Business process rather than what you want to do in SAP.
Thanks & Regards
Sanil Bhandari