How to apply vendor's Unused Credit in EUR to bill in USD?

How to apply vendor's Unused Credit in EUR to bill in USD?

Hi,

Our vendor issues two monthly bills: one in EUR and the second in USD. According to the agreement, we make one total payment in EUR for both bills using the exchange rate stated in the USD bill. 

As we pay in the current month for the services rendered during the next month, the total EUR payment goes to the Vendor Advance in the Banking section during manual categorizing. As a result, we have Unused Credit in EUR for this vendor. 

Part of this unused credit is allocated to the bill in EUR with no problem, so I have to allocate the rest to the USD bill (with appropriate allocation of the currency rate difference if it occurs). 

The question: how can I do it? There are no options to choose the USD bill for EUR unused credit.