It is possible to allocate receipt or credit notes against balances in a customer account.
Allocate Receipts from Receivables Tab for all customers
Allocate Receipts or Credits for a specific customer using the Customer Tab.
Click allocate in the "Actions" field to view all invoices and journals that are outstanding on this customer's account.
If a mistake was made with previous allocations these can be rectified by selecting the previous receipt which was wrongly allocated, all the invoices which the receipt was allocated to will show (together with other invoices which are yet to be allocated to). The amounts allocated against them can be amended at will.