He was also asked:

If someone gives another man a gift of something whose particulars are unknown, or grants him permission to use it, is this valid? For example, what if he allows him to take the fruit of a tree next year? And if he wishes to revoke this, may he validly do so?

He replied:

The scholars have differed over giving gifts whose particulars are unknown. Mālik permitted this, even allowing someone to give another whatever he had inherited from a certain person, without knowing its amount or whether it was a third or a quarter. The same applies if he gives him a share of a house without knowing how large that share is. He likewise permits giving something not yet in existence, such as the fruit of one’s trees for the current year or for ten years. Al-Shāfiʿī did not permit this. The recognized position in the schools of Abū Hanīfah and Ahmad is likewise to prohibit it. However, Ahmad and others permit forms of settlement concerning undetermined claims, and release from such claims, that al-Shāfiʿī does not permit. Abū Hanīfah likewise permits some such arrangements that al-Shāfiʿī does not.

Al-Shāfiʿī requires the amount of the subject matter to be known in most contracts, even the compensation for a wife’s release from marriage, the dower, and the terms stipulated for non-Muslims under Muslim protection. Most scholars allow greater latitude in this regard. This is discussed in its proper place. Mālik’s position on this issue is stronger.

This question is connected to another fundamental principle: contracts of exchange, such as sale, marriage, and compensated divorce, become binding before possession is taken. Thus, taking possession—which follows from and is entailed by the contract—