Paying USD invoices with an account credit paid in GBP

It will create a new Tagged transaction on the bank when you post a credit. You then remove the other one.

Is this a Forex account? If it is then you actually need to create bank accounts for each currency you’re dealing with and rather than tag the payments from the Sterling account, tag as transfers to the respective Forex accounts. You then split up and pay all your invoices from there.

There’s a post here on FX accounts

I should warn you it as a more complex aspect of bookkeeping so will require some time to digest and test the setup.