Sean gives an oral promise to Oscar to buy Oscar's old house,but only offers to keep the promise enforceable for 30 days.Oscar,relying on Sean's promise,puts on a new roof and paints it.Twenty days later,Sean changes his mind and says he's no longer interested.Oscar can enforce Sean's promise.

Sean's promise is enforceable under the doctrine of promissory estoppel. Oscar relied on Sean's promise and acted in detriment by putting a new roof and painting the house. Therefore, Sean cannot back out of his promise without causing injustice to Oscar. The fact that the promise was only enforceable for 30 days does not affect this analysis.