If you had gone to immigration yourself, to renew, you could return to them and get a letter of permission to enter/exit Mexico while your FM3 is in process. Now, you'll probably have to work that out with the lawyer, who should have been able to renew your FM3 in a week. I don't recommend just leaving the country and abandoning your FM3; you should cancel it if you don't have plans to return soon; otherwise, if your plans change, it could be difficult to get another one in the future.