Bava Metzia 16B

משום כדי חייו.

the rabbis instituted an ordinance that the sale is effective because of their concern for his immediate livelihood.

אמר רב הונא אמר רב: האומר לחברו שדה שאני לוקח, לכשאקחנה, קנויה לך מעכשיו ⁠– קנה.

§ The Talmud continues to discuss the matter of selling property that one does not yet own. Rav Huna says that Rav says: With regard to one who says to another: With regard to the field that I am about to buy, when I buy it, it will be retroactively transferred to your ownership from now, the stipulation takes effect, and once he buys it, the second party has acquired the field.

אמר רבא: מסתברא מלתא דרב בשדה סתם. אבל בשדה זו ⁠– לא. מי יימר דמזבין לה ניהליה.

Rava said: The statement of Rav is reasonable only with regard to an unspecified field, as one is capable of buying a field. But with regard to a case where one says to another that he is selling him this specific field that is not yet in his possession, the transaction does not take effect, as who is to say that the current owner will sell it to him? Since it is not in his power alone to purchase the field, it is tantamount to an entity that has not yet come into being, and therefore he cannot sell it to anyone.

והאלהים! אמר רב: אפילו ב״שדה זו״ מכדי רב כמאן אמרה לשמעתיה ⁠– כרבי מאיר, דאמר: אדם מקנה דבר שלא בא לעולם.

The Talmud emphatically rejects Rava’s qualification of Rav’s statement in the form of an oath: By God! Rav said his statement even in a case where the seller said: This field. After all, in accordance with whose opinion did Rav state his halakha? It was in accordance with the opinion of R' Meir, who says that a person can transfer ownership of an entity that has not yet come into the world.