Exchange Rate fixed in PO but still difference in GRN

Dear All,

We are using exchange rate indicator fixed in Purchase Order but still value for custom duty in GRN is calculated as per the exchange rate maintained in OB08 for the date of posting the GRN.

PO created 31.03.2015 with document date as 30.03.2015 so that exchange rate is picked for previous date and fixed indicator ticked.

Custom invoice posted with exchange rate as in PO.

GRN posting date - 31.03.2015

But while posting GRN, custom duties are calculated as per exchange rate on 31.03.2015 instead of exchange rate maintained in PO.

Custom duty value 333.40 USD is calculated as 307.75 in GRN.

Regards

Vikas Sehra