First, appeal the overpayment. You need to appeal to the Administrative Law Judge level, asking that recovery of the overpaid amount be waived.
Here is a brief legal lesson:
Waiver of an overpayment depends on whether (1) the overpaid person was "without fault" in connection with the overpayment, and, if so, (2) whether adjustment or recovery of such overpayment would either defeat the purpose of Title II of the Social Security Act or be against equity and good conscience.
A person cannot be found to be "without fault" if the overpayment of disability benefits resulted from one of the following: (a) the overpaid person made an incorrect statement which he or she knew or should have know was incorrect; or, (b) the overpaid person failed to furnish information which he or she knew or should have known to be material; or (c) the overpaid person accepted a payment which he or she knew or could have been expected to know was incorrect.
However, even though a person may be "without fault" in causing or accepting an overpayment, 20 C.F.R. § 404.506 (code of federal regulations; just type in the citation in google and you will find it) states that repayment must be made anyway unless recovery would defeat the purpose of Title II of the Act, or be against equity and good conscience.
“Against equity and good conscience” means:
(1) that the overpaid person changed his or her position for the worse or relinquished a valuable right because of reliance upon a notice that a payment would be made or because of the overpayment itself; or
(2) the person was living in a separate household from the overpaid person at the time of the overpayment and did not receive the overpayment (both are probably not applicable here).
Instead, most people who get to this step prevail on the 'defeal the purpose of Title II" part.
To defeat the purpose of Title II means that adjustment or recovery of the overpayment would work a financial hardship by depriving the person of income required for ordinary and necessary living expenses.
So, appeal the overpayment. Simply write on the appeal “I feel that I was not overpaid or that I should not be made to repay the overpaid amount” (something like that). If you receive yet another letter from the Administration saying that they have reviewed your case and that you still must repay the overpaid amount, appeal again to the Administrative Law Judge.
Make a list of your monthly expenses. Find or keep all the receipts you can (utilities, mortgage, etc). You may also list a reasonable monthly amount for food, clothing, etc.
If you decide to hire an attorney or disability representative, look for one who specializes in this type of law. You will have to pay (unless you have a local free legal society). Do not panic if you can’t find a representative! I have seen many people represent themselves in front of the judge and win the case. Most Administrative Law Judges are fair people, and many tend to be suspicious of the Administrations claim of having discovered that someone was overpaid benefits. The Judge can also, in the worse case, order reduced monthly payments.
Good luck.