You need to get a Permiso de Salida y Regreso; which is basically just a letter saying that your FM3 is en trámite and you are allowed to leave Mexico and return using the letter itself as your exit and entrance document. As RVGRINGO said, it costs around 250 pesos. You can probably find a list of the documents you need to apply for the letter if you search the website www(dot)inm(dot)gob(dot)mx for "Salir de México permiso de salida y regreso".

Basically you need the usual identification documents (passport), the copy of the application form for your pending FM3 process (i.e. the receipt they gave you or your lawyer when the submission for the process was made), any documentation you might have showing your need to leave the country at this time (airplane ticket [bus ticket in your case?]; possibly any documents you might have about the bereavement, if any), the completed payment form showing that you've paid the appropriate fee at the bank, and a letter you've written attesting that you aren't subject to any pending legal process (i.e. you're not fleeing arrest). And of course copies of everything.

Your lawyer ought to be able to help you with this. If you can get this letter, there should be no need either to cancel your pending FM3 process or run the risk of using your expired/canceled FMT.