Issue customer refund when they only have fully paid or negative invoices

All invoices paid – Create cash receipt from paid invoices

1. Unapply cash from an invoice with an amount greater than the refund amount.
2. Enter a Billed Credit or AR Credit adjustment against the invoice for the refunded amount.
3. If the adjustment entered in Step 2 was the full outstanding amount, skip to Step 4. If the adjustment is less than the full outstanding amount, apply the cash receipt to the invoice again.
4. Refund the customer cash receipt.

Negative or credit invoices – Create cash receipt from credit invoice

1. Unapply cash from a positive invoice.
2. Select the Customer cash receipt and click Apply.
3. Select the negative invoice, then the positive invoice, and click OK.
    NOTE: Click Unapply to de-select any invoices if they're selected automatically. 
4. Refund the cash receipt to the customer.

How do I refund a cash receipt or overpayment to an Accounts Receivable customer?
How do I adjust or write off an invoice in Accounts Receivable?