Debate an overpayment

Ask for the support or back-up, as well as an explanation, for why they claim you are overpaid so much; review it for yourself and see if you feel they are correct or not. It should be easy to determine, one way or another: you should know your wages or salary (i.e. what you are paid per hour or per week0, and also the hours or days worked (or be able to get this information, such as from time cards): if you know how much you worked and what your wage or salary was, you can tell what you *should* have been paid. Then compare what you should have received to what you did receive; that will tell you how much you were overpaid by.
If you end up agreeing with your employer, you will have to repay that amount--the law is clear that an accidental overpayment does not let you keep the money. If you disagree, try to work it out with them--show them why they are wrong. If you and they cannot agree as  to the amount, they could try to sue you for the repayment if you won't volutntarily provide it, but would only get the money if they can convince a judge that they were write and you wrong; the law does not let an employer take money from an employee other than with employee consent (agreement) or as ordered by a court.