If you have an overpayment, we'll send you a decision letter. It will say how much you owe.Ask them for an application to waive the overpayment. You must pay it back because you were at fault for the overpayment. We hope it helps you resolve your overpayment as quickly as possible. Element 4: Applying estoppel is necessary to stop a clear injustice. You must show 2 things to meet this element: 1.