I'd at least give it a shot. For the gas pedal, you can just unbolt the entire assembly and swap it from one car to the other - it is held on by one bolt and then you just have to remove the throttle cable (very easy swap). While the clutch and brake may be hard, at least try taking one off and see if you can get it on the other car (maybe work your way from the bottom where it is narrower and try to slide it off one end). When installing, just make sure to use some soap or other substance and just keep playing with it until you get it on, it may be easier actually since its already been stretched. I have installed two sets on both of my Mustang's (current and past), and was able to do it by taking my time as well as going crazy on the pedal, haha - use a mix.
If you cant get it on, just say the pedal slipped off on the way to the dealer and have them reinstall it! I think its worth a try for $50 - to me it would be at least. Or if you are selling private party get it back on as best as you can and let the buyer know it slipped off a little, but you should be able to do it.