Answer: She is not.
Step-by-step explanation:
It would seem as though that Mary got into a type of contract known as an Option Contract or more precisely, a Call Option Contract simply called a Call.
In this type on contract, a seller gives a buyer the right to buy a good or service at a certain price within a set period.
Mary agreed to sell the rare Chinese Art for a certain amount which Mike could not pay but she promised to give him 3 weeks to take it within which he can pay and collect the item.
Mike returned in 2 weeks which was within the range of time allowed and so she should have kept the offer open for the time she said she would.
She is wrong to believe that all she owes him is his down payment. She broke a contract.