You can't record the payment; a message says the transaction is unbalanced
Before you can record a customer payment, the payment amount must be fully allocated. That is, the total amount of the payment (as shown in the receipt image at the top of the window) must be assigned to the invoices that are being paid. For example, if the customer is paying for invoice 1001, locate that invoice and click in the Amount Applied column to apply the customer's payment to that invoice. Customer payments can be split between two or more invoices.
When the payment amount is fully allocated, the Out of Balance amount near the bottom of the window will be £0.00, and you'll be able to record the transaction.