How do I allocated a customer payment that has come through the bank feeds to an invoice that is already marked as paid
If the invoice has already been marked as paid, you will not be able to match the received funds on the bank feed to the invoice as it's considered closed. If you mistakenly received the payment to the invoice, you can delete the received payment, and then the invoice should show as an available match.
After performing all these steps, you can now reconcile the account seamlessly. For future reference, I’m adding a link that will guide you on how to handle a customer refund: Record a customer refund.
Feel free to visit the Community if you need help with QBO. I’ll be happy to assist further. Have a great week ahead.