Greg and Tina discover a ring buried in Greg's backyard.They both think it is a cheap piece of junk.Tina wants to have it and Greg lets her buy it from him for $2. Tina takes it to a jeweler, who tells her it is a real stone set in real gold worth $200.Tina tells Greg the good news and Greg wants to cancel their agreement to sell and get the ring back.Do you think Greg can get the ring back? If so, what theory can he use to do so?