Nine years ago, a retired jockey deeded a large parcel of land to a charitable organization. The warranty deed stated, in part, that the property was transferred to the organization "provided that the organization uses the property as a facility for teaching children about thoroughbred horses, otherwise [jockey] may immediately retake the land." Three years ago, the charitable organization turned the facility into an organic farm. The retired jockey, who had become very interested in organic farming, learned of the plans to make the change but did not object. Recently, the organization entered into negotiations to sell the property to a large farming corporation. May the retired jockey stop the sale?