BLAW 2110 Business Law

Before Bill starts college, and his grandparents promise to pay up to $15,000 of his student loans if he graduates. He goes to college, borrowing and spending more than $15,000. At the start of his last semester, Bill reminds his grandparents of the promise. They email him and say, "We revoke the promise." Is the promise by Bill's grandparents to pay up to $15,000 binding if he graduates?