If your supplier utilizes different currencies, you can add their currencies to the 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 currency of the supplier will be visible both on the PO page and in the general list of orders.

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