Question: A man bequeathed to another man a waqf for him and for his progeny after him. The legatee then wrote a will that the yields be split in half: half of the yields to his heirs, and the other half to a man unrelated to him. What is the ruling?
Answer: The waqf belongs to Allah. The man who was bequeathed it, then his progeny, is entitled only to the usufruct and the yields from the waqf during his lifetime. When he dies, his right ceases and the heirs deserve the yields. If he then bequeaths the yields to his heirs and to another unrelated man, his bequest has no legal effect—because his heirs already deserve all of it by the first founder’s bequest; he has bequeathed what he did not own.
Source: Min Thimār al-ʿIlm wa al-Ḥikmah vol.2