Hi there, @userdora-mahan.
I can help walk you through the steps on how to correct the invoice payment amount in QuickBooks Online.
Let's create a Check or Expense to record the refund. This will reduce your bank's balance and offsets the customer's overpayment.
Here's how:
- Go to the New icon, then choose Expense or Check.
- Choose the customer's name in the Payee drop-down arrow.
- From the Payment account drop-down, pick the bank account where you deposited the overpayment too.
- Choose Accounts Receivable on the first line, then enter $20 in the Amount field.
- Hit Save and close.

After that, let's link the refund you've created earlier to the customer's overpayment. Let me guide you how:
- Go to the Plus icon, then choose Receive payment.
- Pick the customer's name in the drop-down.
- Complete the needed information.
- In the Outstanding Transactions section, put a checkmark for the transaction you created.
- Under Credits, choose the invoice payment.
- Tap Save and close.


For the detailed guide, check out this article: Record a customer refund in QuickBooks Online.
You can always open customer's reports to review the transactions, then customize it to show the specific details. Also, saving the customization report is helpful too. It will help you save time since it automatically updates every time there are newly added transactions.
I've also added an article about managing your customer's refund, credits, payments, sales forms, and many more.
Let me know if you have follow-up questions. I'm always here, ready to help. Take care!