Exchange rate difference in to different G/L account

Dear All,

Ex: I have raised a PO for Imports with account assignment category "Q",during creation of purchase order the Exchage rate was 46.65 INR. when i'm doing Invoice MIRO the exchage rate is 48.85.The difference is going against WBS element.But now our company wants the difference in currency should be against cost center

Please can any one suggest how can i hit Exchage rate difference against cost center.

Regards

subhash