Joe drove his car into Sam's garage to have some repair work done. He told Sam to fix the carburetor. Sam worked on it most of the afternoon and found he had to rebuild the carburetor. He presented Joe with a bill for $250 when he returned. Joe refused to pay, saying that he had taken a law course and knew that, because they didn't agree to a specific consideration ahead of time, he was not bound in contract. Was he correct? Explain.