If there is an overpayment on a project, it will be displayed in the Billing section of the project. The amount will be made clear and recorded at the top.
There are a couple methods you can use to fix this issue
Method 1: Add a custom item to the project for the amount they overpaid.
Method 2: Provide the client with a refund (if items/services were removed).
To learn the difference between a refund and account credit click here.