Is it permissible to expose someone who sends financial aid to poor students and then seizes it, knowing that he is a sheikh and leads prayers in the mosque, and that he does not openly commit his sin?

As long as the person is not openly committing sins nor known for corruption, it is not permissible to expose them. Rather, they should be advised and admonished while their faults are concealed. If they persist in their sin, then it becomes permissible to warn against them to the extent necessary, such as warning those who entrust them with their money.