If in fact this is a request to recover an alleged overpayment you need to submit to Social Security a "Request for Reconsideration" as well as a Request for a Waiver. Both of these forms are in PDF format on the social security website under forms. These must be submitted within 60 days of the receipt of the overpayment request. I suggest sending these forms to your local social security office and do so by way of certified mail return receipt request. You will then have proof of your submission. Also, keep a copy of the forms you complete for your records.