אין אדם מקנה דבר שלא בא לעולם.
One can not cause to acquire something that is not yet in one's possession. Yevamot 93.
R Meir says one can and this seems to depend on Aristotle's idea of the sea battle that will take place tomorrow. That is though he can not cause it to be acquired now but it is true now that it will be acquired.
Rav agrees with R. Meir if he says "from now." There is a three way argument between Tosphot, the Rashba and the Ramban what that means.

See Rambam laws of Acquisition 22:1