I'll make sure you can adjust the additional credit payment in QuickBooks Online (QBO), usermaw. 

You can create a cheque or expense for the customer refund affecting accounts receivable (debtors account). Once done, link it to the credit note using Receive Payments. You can follow the steps provided by my colleague above on how to create a cheque.

I've added screenshots for your reference. 

Additionally, here's an article that provides tips on handling a customer credit or overpayment in QuickBooks Online: Learn what to do in QuickBooks Online when a customer overpays you.

That helps you out! Tap me on my shoulder if you have more concerns about applying for credits in QuickBooks Online. I'll be around to help. Take care and stay safe.