✔ 最佳答案
As the advertisement is a public document, theorically speaking, he is obliged by his promise.
However, unless you asked him for $100 at the time you gave back the book to him or have a receipt from acknowledging the lost book from you, there is no evidence that the book was found by you and delivered to his hands.
So, in principle, it is hard for you to claim back $100 from him. But, you can ask him for a friendly reward or as a matter of jesture for something.