Hi Lokesh,
While making Partial Payments also you can post forex gain/Loss but for that a little configuration is required.
The IMG path is :-
Financial Accounting Global Settings (New)>Global parameters for Company code>Currencies>Indicate Currency Differences for partial payments.
Here against the document type, which you are using for making partial payments you have to tick that doc type and then the Forex gain/Loss gets posted even on partial payments.
Regards,
Kavita