אין אדם מקנה דבר שלא בא לעולם.
One can not cause to acquire something that is not yet in one's possession. יבמות צ''ג.
ר. מאיר  says one can and this seems to depend on Aristotle's idea of the sea battle that will take place tomorrow.  Is it true now? Or is it true only when the sea battle takes place?

Though he can not cause it to be acquired now,  but it is true now that it will be acquired.
רב agrees with ר. מאיר if he says "מעכשיו". There is a three way argument between תוספות, the רשב''א and the רמב''ן what that means.

This way of mine seem to fit with how רב שך explains תוספות and the רשב''א.
See רמב''ם laws of מכירה כ''ב:ה''א