voucher match with PO and foreign currency

What does the box Foreign currency do if you uncheck it when voucher matching but the PO is in a foreign currency and received that way.  We experienced that the voucher match process if we uncheck the box it still writes the foreign currency $ amount but the currency code is coded as the base currency.  Then we have posting issues because the CA is out of balance.  What is the purpose of the Foeign box on voucher match and what is the correct way is using this