Can a lawyer keep attorney fees on an SSI case, even if they worked on contingency?

I hired a lawyer to represent my son's SSI case in front of a judge for the final step in the appeals process. We lost the case to continue his disability. However, during the time we were waiting on our appeals case, I had opted to continue receiving benefits. We paid the lawyer close to $1,000 dollars to represent us. When the decision came down from SSA that he no longer qualified, we received an overpayment notice of close to $24,000. We filed a request for waiver of overpayment and were approved. I sent this letter to the attorney and asked if she would return the money paid to her since she lost the case. She is now telling me that she won't return the fees, even though she worked on contingency. How can I go about getting my money back?