Thanks for your reply, @Von-1. Let me provide additional information on handling customer refunds in QuickBooks Online.

If the invoice has been paid and your customer wants to reduce their current balance immediately, we can utilize the adjustment note. I'll show you how: 

1. Go to + New and select Adjustment note.
2. From the Customer field, choose the appropriate customer.
3. Enter the Adjustment Note Date, Amount, GST, and Product/Service (This is the category, product, or service you're getting credit for).
4. Click Save and Close.

Otherwise, here's how you can refund an invoice that your customer has already been fully paid for: 

1. Click + New and select Refund receipt.
2. Choose the customer you want to refund from the Customer ▼ dropdown.
3. Enter the details in the fields using the info from the original sale.
4. Select which payment method and account you wish to refund the customer from.
5. Tick Save and Close.

To learn more about it, please review this handy guide: Handle customer refunds in QuickBooks Online.

In case you'd like to send your customer a summary of their invoices, payments, credits, and balances. See the following article for detailed guidance: Create and send customer statements in QuickBooks Online.

You can always reply or reach out to us again if you need anything else with transactions in QBO. We're always here to guide and assist you whenever you need help.