Kindly clarify the meaning of the ruling: “One cannot sell a fruit before it's even ripen/ almost ripe” and also the concept of Salaam.

Let's say I have orange trees which are not yet ripe; I cannot sell these fruits (let’s say 3 bags which weigh 50kg each) to you in that condition but we can agree that you pay me in advance and then when the fruits are ripe I will then deliver them to you on a specific date and time. In such a case the transaction will be valid.

This is what is referred to as salaam. By principle it was supposed to be an invalid transaction because of the ambiguity surrounding the commodity.

Salaam is an exception to this because of the people's need toward such a transaction and the fact that the commodity is unambiguously defined and the date of delivery is determined and agreed upon. [1]

Reference:

[1] Sheikh S Al-Fawzaan, A Summary of Islamic Jurisprudence, 2/59 - 63