If you purchase from a vendor in one currency and submit payment in another currency, you can apply the payment to the purchase.
If you apply an entry (Entry 1) in one currency to an entry (Entry 2) in a different currency, the program uses the posting date on Entry 1 to find the relevant exchange rate to convert amounts on Entry 2. The program finds the relevant exchange rate in the Currency Exchange Rates window.
Open the 
Payment Journal window.
Fill in the first journal line using a currency code. For Help about a specific field, click the field and press F1.
Click Functions, Apply Entries.
Click the line with the entry you want to apply to the entry in the payment journal. Then click Application, Set Applies-to ID and select the entry you want to apply to.
Click OK to return to the payment journal.
Click Posting, Post in the payment journal to post the payment.
When you apply entries in different currencies to one another, the program converts the entries to LCY. Even though the exchange rates for the two relevant currencies are fixed, for example between DEM and EUR, there may be a small residual amount when the program converts these foreign-currency amounts to LCY. The program posts these small residual amounts as gains and losses to the account specified in the Realized Gains Account or Realized Losses Account field in the Currencies window. The program also adjusts the Amount (LCY) field on the relevant vendor ledger entries.
Allowing for Application of Vendor Ledger Entries in Different Currencies
Allowing for Rounding Differences when Applying Entries
Unapplying Applied Customer or Vendor Ledger Entries