There may be instances where a client has a credit balance and you wish to allocate this as part, or full, payment to an invoice that is due.
When allocating credit as part payment of an invoice, with an additional payment e.g. cash
+Add payment - Insert the received amount (excluding the credit amount) - this has to match the allocated amount - it does not have to match the invoiced amount.
Allocate the credit (or part thereof) by selecting "Apply credit" and then insert the amount you are allocating to this particular invoice. It is possible to allocate a payment to more than one invoice.
If you are allocating credit as part payment, with no additional received amount, simply allocate the credit to the invoice and save. You can come back to the invoice and add an additional payment amount, when you receive the payment.