exchange Rate diff

Hi experts

I have to post an outgoing payment in advance in usd, & the invoice is posted few days later,

I m giving u details of the scenario

advance payment made on 01-05-2010 4000 USD @ 50 = 200000 Rs

invoice raised on 10 - 05 - 2010 4000 USD @ 40 = 160000 Rs

How to post the exchange rate diff in this scenario.