MS GP - How to apply RM docs with differemt currency IDs?

Using multicurrency and applying docs in RM can be tricky, but here is the basic system functionality:
If the invoice currency ID is entered in an originating currency, the currency of a payment must be the same as the invoice currency or a functional currency. If the invoice is in the functional currency, a payment can be entered in any currency. If you have an invoice and a payment, both created in 2 separate originating currencies, you will not be able to apply these documents.