We are doing a voucher to Purchase Order match and want to use GL date for currency exchange

we receive in the PO and then we do the voucher match and this is where we want to control the exchange rate date.  Also when we then do a manual or automatic payment where can we control the exchange rate date on the voucher match. I thought it was on the p4314 processing option #8 for currency and also the Master business function P0400047 under currency I have changed these but it is still pulling in today's exchange rate not the GL date's exchange rate