All perfect praise be to Allah the Lord of the Worlds. May His peace and blessings be upon our Prophet Mohammad and upon all his family and companions.
The Juzaz (olive harvesting arrangement) is a contract between the owner of the olive trees and the worker, where the worker is responsible for harvesting the olives only during the season, without being involved in the care of the trees. The default principle in Ijarah (leasing) contract is that it should be a known and predetermined amount, and it should not be a percentage of the yield, as per the Shafi'i school of thought. However, the Hanbali scholars have allowed the rent to be a share of the yield. For such an arrangement to be valid, it is necessary that the share is determined in proportion to the yield, such as half for half or one-third for two-thirds, and so on.
If the owner of the olive grove stipulates that he will take three oil tins after the extraction process, this is not valid according to Islamic law (Sharia). This is because it involves uncertainty and ignorance, as the quantity of oil may be less than the specified number.
Imam al-Buhuti, may Allah have mercy on him, said: "It is valid to provide a servant, an animal, or a plowing tool to someone who works with them in exchange for a portion of their wages. It is also valid for sewing a garment, spinning yarn, harvesting crops, or using someone's property for a portion of its shared benefit. This is because it represents an asset that grows through labor. Therefore, a contract is valid based on a part of its growth. For example, trees in Mosaqah (A type of partnership in which the owner of an orchard agrees to share a stipulated portion of the produce of the orchard's trees with a worker, in exchange for the latter's irrigation of the garden) or Muzara`ah (A partnership in crops whereby one party (the landowner) presents a piece of land to another (the agricultural worker) for the latter to cultivate and maintain in return for a common defined share in the crop).
This does not contradict the hadith reported by ad-Daraqutni: "He (peace be upon him) forbade the hiring of a stallion and the millstone." This is due to the uncertainty of the remaining amount after grinding. The benefit becomes unknown, even if a dirham or more is added to it, the contract is not valid according to explicit evidence." [Dakaiq Uli al-Nahy li Sharh al-Muntaha, Vol. 2/P. 228].
In conclusion, if the owner of the olive grove agrees with the worker on harvesting and pressing the olives in exchange for shares in proportion to the yield, such as half or one-third, that is permissible according to Islamic law. However, if the owner agrees to receive a specific number of oil tins, such as two or three, this is not permissible according to Sharia due to the presence of uncertainty and ignorance. And Allah the Almighty knows best.