What to do when the mechanic fix something that wasnt broke?

i took my car to the shop and the mechanic told me that he would pinpoint my cars problem,because it wasnt starting.before i took it in i talk about the problem with other mechanics and most of them told me that i didnt need a fuel pump that my problem was in the injection,i relayed this information to the mechanic,he said it was possible but he didnt think that the injection was my problem,so he insisted that i replace the fuel pump,i agreed.my car is still not running ,now hes telling me that i need to pay him first for the fuel pump before he fixes the injection. i dont want to pay for him fixing something that wasnt broken.what should i do?