If your supplier utilizes different currencies, you can add their currencies to the profile. You can do that on the Suppliers Management page.

add currencies to the suppliers' profile

And then, when the PO is created, you can choose the currency you need to use and place an accurate order. 

The order amount in the supplier's currency will be visible on the PO page and in the general list of orders.

The exchange rate for a foreign supply order is only used to estimate the preliminary cost of the order. But this is not the final rate to be used for payments.