You can use the AR Payment on Account form to apply payments to an account balance rather than individual invoices.

You can access this form by clicking On Account on the AR Cash Receipts form or by selecting the On Account option on the AR Initialize Receipt form.

Payments applied 'on account' are useful when you receive a payment from a customer before they have received an invoice or when you are unsure how to apply the payment. By applying payments on account, you can effectively hold the payment until you are ready to apply it to the appropriate invoice(s) later. To apply 'on account' payment to an invoice, you must reverse the 'on account' payment and reapply it to the appropriate invoice.