Chu, a non-swimmer, fell off a ferry boat into the lake. "Help!" he cried. "$500 to the person who saves me!" Frances, a passenger on the ferry, then jumped in and held Chu's head above water and pulled Chu to shore. Once Chu and Frances were on shore, Chu refused to pay Frances. Please advise Frances whether Chu can refuse to pay Frances by referring to relevant concept(s) in contracts.