I wouldn't call this "clean", but:
1. USD JE to move the balance into a clearing account; DR AR, CR Clearing/Holding
a. Need to specify customer on the AR line
2. GBP JE to move the balance out of the clearing account; DR Clearing/Holding, CR AR
a. Need to specify customer on the AR line
b. Might have to futz with the FX rate to ensure it gets recorded in an amount that actually clears the clearing account
c. if you don't, the accounting team will be mad. 🙂
3. Apply JE from #1 to your existing CM
4. Apply JE from #2 to the invoice they want to apply to
If 2/4 is going to screw up remittances/statements, etc, you could have 2 be a credit memo transaction, but you'd have to have an item that has the clearing/holding account as its GL account.
Also, this is obviously a once-a-month type process, hopefully not 10 times a day. If you are doing this constantly, I'd set up some type of automation.