The father of a young child whose life was saved by doctors at a hospital emergency room called the president of the hospital and told her that he will donate $50,000 to the hospital, payable in 90 days, in consideration of the doctors' saving his child's life. The president recognized the father's name as a wealthy philanthropist who had donated generously to other worthy causes, and accepted the offer. The president promptly informed the hospital's board of directors of the father's offer and her acceptance of the offer. The board approved the purchase of a new respirator for $50,000 and authorized the president to enter into a written contract with a medical devices company to make the purchase, which she did. Once the child recovered, the father changed his mind and refused to give the hospital the $50,000. If the hospital sues the father, what would the hospital's best theory be to recover the $50,000