Mary offered to sell Mike several pieces of rare Chinese art at a very good price because they were duplicates in her own collection. Mike could not accept the offer at that time, but he did give Mary $500 in return for her promise to keep her offer open for three (3) weeks. Mike returned with the agreed-upon balance two weeks later to find that Mary already had sold the pieces she had offered to sell to him. Mary explained that she had been able to get a better price from another buyer. 

She offered to return Mike's $500 and insisted that this was all she was obligated to do. Is Mary right?