No. There is no law that says they have to give you an accounting. It would be a nice thing to do, but the bottom line is if they are right, you will have no defense to their adverse action by complaining that you were not given an accounting.
Presumably you know what you were owed. If you are salaried, you know what your salary is. If you are hourly, you presumably know how many hours you worked and at what rate of pay. If you are piecemeal, ditto. Take your numbers and compare the their. Do not just sit back with their money in your pocket and say "prove it or I wont repay." That is a recipe for disappointment.
Good luck to you.
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You are entitled to pay stubs that reflect the number of hours you worked and the hourly rate at which you were being paid. If you work on commission basis, you are entitled to a written commission agreement which indicates how your commissions are to be calculated and when they vest.
You are not entitled to any sort of "explanation" beyond this. If your employer overpaid you then you have to give that money back. You can be sued and/or fired if you don't.
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I agree with my colleagues that if you actually owe the money, the employer can pursue you for it. However, the employer would have to seek recovery through a small claims or civil action against you as the employer is not entitled to what is called "self help" by deducting the amount they claim you were overpaid from your future earnings.