Hazrat Tawus says that when the quantity of something is known, then do not sell it by estimation.

Hazrat Mu'tamir bin Sulaiman narrates that I said to my father that a person said: "I have weighed this pot, it contains such-and-such man (a unit of weight), but I do not know, perhaps it has decreased, or some of it has been stolen, or it has been mixed with another pot, or there has been some mistake in it. I will not sell it by exact measure, I will sell it by estimation." What is the ruling on this sale? He replied that Hazrat Ibn Sirin disliked it, while Hasan (may Allah have mercy on him) did not see any harm in it.

22761 - And Al-Hasan saw no harm in it.

Ibrahim (may Allah have mercy on him) was asked about a person who buys by estimation and says: "Whatever wheat you have in your house is for such and such amount, and whatever barley there is, is for such and such amount?" Ibrahim (may Allah have mercy on him) disliked this.

Hazrat Sha’bi (may Allah have mercy on him) was asked that some villagers brought grain to us, we measured and bought some from them, then they said,“Make the sale with us by estimation?”He replied,“Do not do so until they agree to leave the sale.”

Hazrat Ata (may Allah have mercy on him) did not consider there to be any harm in selling by estimation, as long as the quantity of the item is known.

Hasan, Hazrat Mujahid, Hazrat Ikrimah, and Hazrat Ata were asked about a person who goes to another and sells wheat by estimation, and sometimes the quantity of wheat is also known, so what is the ruling on such a sale? All the respected individuals disliked it.

It is narrated from Hazrat Nafi’ (may Allah have mercy on him) that I saw the companions of the Messenger of Allah (peace and blessings be upon him) that sacks of grain were brought before them, and a man would say that I have measured these things and bought them, but I will sell them not by measure but by estimation (without measuring). The companions of the Prophet did not consider there to be any harm in this.

It is narrated from Hazrat Ibn Umar (may Allah be pleased with them both) that we used to meet riders and buy wheat etc. from them by estimation. The Holy Prophet (peace and blessings be upon him) forbade us from this until we transferred it from its place.