4. Joe offered to sell Harry his car for $500, but Harry said he needed some time to think about it. A couple of days later, Harry phoned Joe to say that he'd give Joe $450 for the car. Joe refused to take this. Then Harry said, "All right, I'll accept your offer for $500." Joe said that he'd changed his mind and refused to go through with the deal. Harry sued. Explain Joe's legal liability.