There is an argument of one can give or gain possession of something that has not come into the world.
R Meir said one can. However the sages said "no", and so throughout Shas, you see it is a given that one can not.
[There is a certain order among the tenaim/ authors of the Mishna with whom is the law. R Jose, R Yehuda, etc. according to order. R. Meir is near that bottom unless it is a "stam mishna" [no authorship is attributed] in which case the law is like R. Meir. [That is how R Yehuda the Prince arranged the Mishna]
In the case of a fruit tree, if one sells it to one person and sells its fruit to another, the other has acquired nothing except fruit which is on it right now. Not anything that will grow in the future. But in a case where he sells the tree to one person and he says, "I am selling to you the tree, but keeping the fruit for me," he keeps the fruit --for it is considered as if he kept the place where the fruit is growing for himself.
Same with a sell of a house where he says I am keeping the upper porch to be able to build upper extensions into the courtyard. But in both cases, there is an argument among rishonim if he can pass that right along to the people that inherit him. The Gra and Rashbam say no. The Ramban [Nachmanides] says yes. The issue is that the right to build an extension is thought to be a thing that has no substance.
The Gemara there in Bava Batra says the case of the Levi who sells his land "on condition" that the first tithe he gets. That arrangement does not continue with his children that inherit him. The idea is he keeps in theory the actual ground that the tithe grows on.
From there Reish Lakish learns from there about a person that sells his house on condition he keeps the roof space. But he keeps it anyway in the ancient usage of Iraq when if one sells a house the seller keeps top of the roof unless that is specified. To the Rashbam saying openly "I sell you the house on condition the rood space is mine" means he added a condition that was implicit anyway. So it comes to include teh right to extend the rood to the other side of the courtyard and to make a walk way there.
To the Ramban [Nachmanides] that is not because of the language but part of the actual arrangement in any case
The Gra holds like the Rashbam that the case of a Levite and roof are similar in that the children do not inherit the right, but the case of the roof is because of owning a thing that has no substance, not because of the language used in the deal.
