Bank Rec - Customer overpayment

Good morning

This is the first time this has happened and I need some help please.  I have a customer who has paid a large amount twice but wants it kept for future invoices.  I have created a negative sales invoice which is now sitting in Customer Returns.  

How do I get this to reflect in the bank reconciliation?

I have a few current invoices I can allocate this payment to - do I go to the credit and apply the invoices that are currentz?  Will this leave the balance as a credit for future invoices?

Thank you in advance :)

Kind regards