Document currency error

Hi Experts,

While doing purchase order or any document, system will not consider Doc rate (OPOR).it consider only Exchange rates and indexes for USD=50 which client not required .client wants only document rate .

example-Doc rate=50 and exchange rate=51 then it consider 51 as doc rate for calculation.so pl tell me why did it happens.

Rgds

Sudhir Pawar.