If the currencies are different, no. Unless you add the prepayment from the supplier / client record directly. In which case, you’re able to specify what invoices it can be used for if the currency isn’t GBP.
If you go to their record, you can click on ‘More Options’ and select ‘Add a Prepayment’, which will present you with the options similar to this:
You can specify the currency by changing invoices it’s available for. By using the options above, it would create a £100.00 transaction, tagged to the client or supplier (whichever you choose), and create the credit in the currency you selected (in this case, EUR). That will then be available for any EUR invoices.
From the screenshot above, €120 is now sat on the suppliers account, but tagged to a £100 transaction in the current account:
