A few years ago, the newspapers were full of stories about a branch of the University of California. An alumnus had made a huge gift via his estate, which consisted mostly of stock in his company. After his death, the stock tanked, and his widow refused to pay off on his pledge. So the school sued her. Normally this occurs when commitments have been made for construction of buildings, hiring of faculty, and the like.
Pledges are legally enforceable commitments and the courts have sided with campuses in the past.
Obviously, this is an extreme situation and, fortunately, not one that occurs frequently in my experience. What normally happens, short of going to court, is that institutions work with donors' families to find a solution that meets everyone's needs. The financial aid decision is based on a number of factors and only a qualified financial aid counselor on a campus can help you with this one. Sports are hugely controversial on the nation's campuses. Many athletes don't graduate, and don't try to.