He endowed a piece of land to build a mosque,
    then changed his mind
Question

                There is a rich person among
      us who donated some land to build a mosque, then someone came along and asked him to make
      it a school. What should he do?
Praise be to Allah.If the man has already donated the piece of
    land for building a mosque, then it should be used for that purpose, and he cannot go back
    on his donation. But if it is the case that all that has happened is a promise to donate
    land for a mosque, it is better for him to keep his promise. May Allaah bless our Prophet
    Muhammad and his family and companions.