This is a typical unilateral contract. You promise to pay for the plumber's performance, which is to fix your hot water problem. He performed, no matter how, and you have to pay, regardless of the mistake you made because it was not part of the contract. A court will not normally examine the adequacy of the consideration. In this case, the consideration is very samll. It only took the plumber two seconds to check it out, but it is still a valid consideration and you must pay him. If you goes to small claim court, you will not only pay the $105 but also the $50 filing fee.