I'll help with this concern so that you can record your customer's payment properly in QuickBooks, @Quickee.
Before we start, I just wanted to clarify about +icon that you're referring to.
If you're talking about the +New icon that you can see in QuickBooks Online, therefore, this isn't available in QuickBooks Desktop. This is the reason why you're unable to see it.
To address your concern, we can create a check to refund your customer. Then, delete the invoice and recreate it with the correct amount. This way, we can make sure your transaction reports are accurate.
Here's how you can create a check:
- Go to the Banking menu, and select Write Checks.
- Select the bank account in the BANK ACCOUNT drop-down list.
- Enter the rest of the information and select Save & Close.
Once done, you can now delete the invoice and recreate it.
Also, you can check out this article to know the different ways of tracking your customer's transactions in QuickBooks Desktop: Get started with customer transaction workflows in QuickBooks Desktop.
Please let me know if there's anything else I can do to be of assistance. Have a great weekend!