Hi Mohit,
In order to help you to understand about FC Valuation, please go through below links:
https://help.sap.com/saphelp_erp60_sp/helpdata/en/13/43c640d8bfb533e10000000a155106/content.htm
http://wiki.scn.sap.com/wiki/display/ERPFI/Valuating+of+Open+Items+in+Foreign+Currency
Julie