Please analyze the following problem. Be sure to state your reasoning.

Superfan attends a live concert by Rockstar and sits in the front row. As Superfan is watching, a lighting tower starts to fall towards Rockstar. Superfan rushes onstage and pushes Rockstar out of the way, saving his life. Later, Rockstar tells Superfan "That was incredibly brave of you and I'm so grateful. I'm going to give you $50,000 for saving me. I hope you'll use it to treat yourself to something special." What will be the result if Rockstar later refuses to pay and Superfan sues to enforce Rockstar's promise? Would it matter if, after receiving the promise and before Rockstar refused to pay, Superfan purchased an expensive new car that she couldn't afford on the assumption that she would receive the $50,000?