Ordinarily, past consideration is not sufficient to support a promise. By past consideration, the courts mean an act that could have served as consideration if it had been bargained for at the time but that was not the subject of a bargain. For example, Mrs. Ace’s dog Fluffy escapes from her mistress’s condo at dusk. Robert finds Fluffy, sees Mrs. Ace, who is herself out looking for her pet, and gives Fluffy to her. She says, “Oh, thank you for finding my dear dog. Come by my place tomorrow morning and I’ll give you fifty dollars as a reward.” The next day Robert stops by Mrs. Ace’s condo, but she says, “Well, I don’t know. Fluffy soiled the carpet again last night. I think maybe a twenty-dollar reward would be plenty.” Robert cannot collect the fifty dollars. Even though Mrs. Ace might have a moral obligation to pay him and honor her promise, there was no consideration for it. Robert incurred no legal detriment; his contribution—finding the dog—was paid out before her promise, and his past consideration is invalid to support a contract. There was no bargained-for exchange.
However, a valid consideration, given in the past to support a promise, can be the basis for another, later contract under certain circumstances. These occur when a person’s duty to act for one reason or another has become no longer binding. If the person then makes a new promise based on the unfulfilled past duty, the new promise is binding without further consideration. Three types of cases follow.