Amanda is a plumber. She is often asked to fix kitchen leaks. When she goes to someone's home, she cannot provide a firm price for her services to fix the problem, because she first must ascertain what the problem is. She bills the client after she diagnoses and fixes the problem. One day, Danny had a leak in his kitchen. He called Amanda to fix the problem. Amanda attempted to fix the leak in Danny's kitchen, but as she was looking around she realized that the job required better tools and more workers. She told Danny she would come back in a few hours. Danny refused, and told her he wasn't going to pay her, because there was no firm price between them and hence no contract. If Amanda sues Danny in court, what legal principle should she rely on?