IF you were in fact overpaid, you have to return it: an honest error, or error by your attorney, does not allow you to hold onto money to which you are not otherwise entitled. (Look at this way: if you were overpaid, you're not losing anything, since you should not have gotten the money, anyway.) You can and should have the law firm walk you through matters carefully, to assure yourself that you were in fact overpaid, before writing anyone a check.