One invoice, multiple payments, two currencies

[R. Victor Klassen]

Here is the situation:

In December, we pre-paid on an invoice of an unknown amount. Paid $630 US, to get $700 credit on future purchases (promotional).

This went on a Canadian credit card, at the then applicable exchange rate.

In January, we made the order itself.  It was more than $700 US, so we paid the difference, on the same credit card, and the new exchange rate.

Now we have one invoice, and two payments.  Entering the invoice is straightforward using the business features.

At the end of entering it, it’s time to post it.  At that point we need an exchange rate.  

Mission control, we have a problem.  

The exchange rate for part of the invoice is different from the rate for the rest of it.  How do we deal with this?