One of the basic questions that comes up regarding issues of ownership in Jewish law is whether or not adam makneh davar she-lo ba la-olam – whether a person can buy or sell an object that is not in existence right now. Rav Naḥman bar Yitzḥak believes that according to Rabbi Akiva a person has the ability to do so, and lists tanna’im and amora’im who follow that approach.
The simplest way to understand this approach is to say that we view an object that is expected – for example, fruits that have not yet appeared on the tree or a purchase that has not yet been completed – to be seen as already in existence from a legal standpoint. Thus, the seller can then transact business with it, either by stipulating that the sale go into effect immediately, or that it take place at some later date. In bringing examples, the Gemara offers both types of cases. Rav’s case has the seller saying “I am selling this field, and when the purchase is complete, it will belong to you from now.” Rav Huna’s case is when the seller says “I am selling you the dates on this tree.” It is clear that the sale will not be complete until the fruit actually appears, which is why Rav Huna allows the seller to back out of the deal – until the fruits appear.
Another case that appears in the Gemara is that of Rabbi Yannai, who had a tenant on his land who paid him by delivering fruit every Friday. One Friday the tenant did not arrive at the usual time, and Rabbi Yannai – relying on the fact that the fruit would be delivered – chose to separate tithes from other fruit that he had in his house so that he would be able to eat the fruit from his tenant on Shabbat. When he turned to his teacher, Rabbi Ḥiyya to ask about this, Rabbi Ḥiyya agreed that his behavior was correct. The proof from this story is that neither Rabbi Yannai nor Rabbi Ḥiyya appear to be concerned with the fact that the fruit had not been delivered. Their concern was whether or not tithes could be taken when the fruit was not all together.