Make it possible to be able to refund funds to a client that has overpaid their account.

When an invoice is issued in credit (e.g.) returned goods are greater than sold goods then this creates an invoice with a credit balance. On the creation of the next invoice there is no way to attach this "credit invoice" to it. So even if the correct payment is taken - invoice total less the credit it appears are if the invoice has not been paid in full. The invoice can not be closed and is still viewable in the Outstanding Invoices report. A short term work around maybe to insert the payment method column into the Outstanding invoices report (assuming there is a payment type of Credit) so that those with outstanding credit balances are immediately recognisable and hence don't need investigation.