Conditions that are not yet extant — Eruvin 37b
תלמוד בבלי מסכת עירובין דף לז/ב
אמר ליה אביי אלא מעתה היו לפניו שני רמונים של טבל ואמר אם ירדו גשמים היום יהא זה תרומה על זה ואם לא ירדו גשמים היום יהא זה תרומה על זה הכי נמי בין ירדו בין לא ירדו דאין בדבריו כלום.
Abaye challenges Rava with the case of a person before whom there rested two untithed pomegranates. The person said: "If it rains today let this one be terumah on that one, but if it does not rain, let that one be terumah on this one." Abaye clearly understands this condition to be a valid condition.
Maharam MeRutenberg (Teshuvos, §939) considers the following case: Reuven had a minor son and Shimon had a minor daughter. Shimon obligated himself in writing to Reuven that when Reuven's son would become an adult, Shimon would marry his daughter to Reuven's son. Shimon stipulated that if neither he nor his daughter would accept Reuven's son's offer of marriage, then he would pay Reuven a specified penalty. In the end, however, Shimon died before Reuven's son reached adulthood, and when Shimon's daughter reached adulthood, she married someone else. However, Maharam exempted Shimon's estate from paying Reuven the promised penalty, as Shimon was prevented from fulfilling his obligation through no fault of his own (אונס רחמנא פטריה).
Maharam rejects the possibility that Shimon's estate is exempt for another reason: Because the stipulated condition involves a scenario that could not possibly take place at the moment — viz., the marriage — because a minor boy cannot contract a marriage. Maharam explains that this is because the concept of "a thing that has not yet come to the world" (דבר שלא בא לעולם) is only an issue when it comes to the object that is the subject of a transaction. For example, in the case in question, if the promised penalty was not extant at the time of the obligation, the transaction would be void (אין אדם מקנה דבר שלא בא לעולם). A stipulated condition, however, need not be extant at the time of the transaction. As proof of this principle, Maharam cites our case, in which Abaye takes it for granted that a transaction may be contingent on whether it rains, or not — a condition that has not yet come to the world.