Alan made a contract with Betty selling her his antique vase for $25,000.When he delivered the vase to Betty, he told her that he would accept $20,000 as he had moved to a small apartment and no longer had the necessary space for the vase.Betty gave Alan $20,000 and her stamp collection valued at $100 in exchange for the promise.Two weeks later, Alan changed his mind and requested Betty to pay him the outstanding balance for $5,000.

Advise Alan of his legal position.