Bukhari

Hadith Commentary:
This means that until its ripeness becomes apparent, salam (forward sale) is not permissible, because this salam was made specifically for the fruits of certain trees.
If someone makes a salam in unspecified dates, then it is permissible,
even if the fruit has not yet appeared on the tree,
or even if the one to whom the salam is made does not possess any trees.
Now, some have said that this hadith is actually related to the following chapter.
Others have said that it is related to this very chapter, and the correspondence is as follows: when salam is not permissible in specified trees despite the presence of the trees, it becomes clear that the existence of the trees has no effect on the permissibility of salam. And if there are no trees, which are the original source of the goods, even then salam will be permissible.
This is the very meaning of the chapter.

Hadith Commentary:
(1)
One form of forward sale (bay‘ salam) is that a person gives an advance payment and says that after a certain period, a specified amount of dates will be required. According to the rules of bay‘ salam, doing so is permissible. In this case, it will not be discussed from where he will procure them, whether he owns an orchard or not. Another form is that a person, at the time of giving the advance payment, says that the dates required are those growing on certain trees in a specific orchard. Doing so is not permissible until the dates have reached a stage where they are fit for consumption. Specifying the orchard and the trees carries the risk of loss and deception; therefore, such a transaction will not be permissible.

(2)
Imam Bukhari rahimahullah wishes to clarify that the seller not possessing the actual commodity is only beneficial when the transaction is for the supply of general fruit. However, when the transaction concerns fruit growing on specific trees in a particular orchard, then even if the commodity exists, the transaction will not be permissible until the fruit has developed the necessary quality. Further clarification of this will follow.

Hadith Commentary: Benefits and Issues: The purpose of "hatta yuhzar" is that an estimation can be made of how much it will be. The yield of the fruits on the trees would be estimated as to how much it would be after ripening. Some have interpreted it to mean that it should be protected and safeguarded. In any case, the main objective is the manifestation of the fruit's ability to ripen, because only after this does the owner make arrangements for its protection and its quantity is estimated.