Kinyan on Davar sheLo Ba L'Olam

My son is learning perek hamafkid. The Mishna allows a watchman to pay for an object stolen from him, in which case he, the watchman, and not the original owner, would collect the penalty of keifel (double-payment) from the thief if caught. The gemara questions how this law works when we know that one cannot aquire a "davar shelo ba l'olam", an item which does not yet exist: the double-payment (keifel) penalty does not exist until the thief is found, and the object no longer exists in the owner's control for that to be sold either. What exactly is this watchman acquiring? The gemara answers that when the watchman originally received the object from its owner there is an implicit sale of the object, contingent on the object being stolen and the watchman later agreeing to pay for it. An interesting chiddush emerges from this discussion: Although one cannot buy a davar shelo ba l'olam, one can buy an existing object on a condition that is lo ba l'olam. In other words, I cannot buy fruit from your trees before that fruit has grown, but I can buy the existing trees on condition that they grow fruit at some future date.