I am a Realtor not a lawyer, but here is my opinion: You can do whatever you want but there may (and probably will) be consequences. If the 1st roofer chooses to, they can take you to small claims court for contractual default so you have to ask yourself if the savings is worth going to court and possibly loosing (it's probably not). Your best bet is to communicate with the 1st roofer to see if they will either match the other estimate or release you from the contract. Or, you could present your case to a lawyer and they may be able to find a loop hole in the contract, but again, would that cost and trouble be worth the savings? Good luck and let this be a lesson not to sign anything until you have shopped it around. This goes just as well for real estate as it does for anything else.