Hi Vasanth
The valuation you do in SAP is normally done with th "reset procedure". This means when you run a valuation the amount is posted e.g. on March 31st and a reset posting is done on April 1st. You do this on every month end closing. If the deal then is due and posted with TBB1 the realized gain/loss is posted with TPM18. There is no shift of unrealized to realized because it was reset.
REgards
Juerg