Exchange Rate at the Time of Clearing

Dear Experts,

We are facing problem with Exchange Rate at the Time of Clearing.

While partially clearing vendor Open invoices System is Calculating Clearing Document (AB) on current Exchange Rate Maintained in (OB08).

We need clearing document Exchange Rate of that maintained at the time of Invoice not at the time of clearing. Clearing is being done in foreign currency.

Regards,

Asim Raza