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If a woman vows that her handiwork is forbidden to her husband, the vow is does not take effect. (1)
If a woman says that her hand shall be Kodesh to her Maker, the handiwork that she has not yet produced is prohibited. (2)
Rav Huna: If a person sells the fruit of a tree to his friend, he may retract as long as the fruit has not yet sprouted. (3)
Rav: If a person gives away a field that he has not yet bought, it is a valid gift.
A person may separate Ma'asros for produce that he has not yet acquired.
Rebbi: If a person buys a servant on condition to free him, he is not allowed to enslave him.
There is a dispute between Rebbi Meir and the Chachamim in the case of Kidushin made on condition that it should go into effect after he or she becomes a Ger. (4)
Rebbi Eliezer ben Yakov: If a person separates Terumah from produce that has not yet been harvested, it is valid. (5)
The testimony of a single witness, or of the wife, that the husband died and that the wife is Zekukah to Yibum, is believed.
A BIT MORE
1. The woman's handiwork belongs to her husband, and thus she cannot prohibit it to him. However, Rebbi Akiva says that the husband still must annul the vow, because perhaps she will do extra work that does not belong to the husband.
2. Even according to the opinion that a person cannot make a Kinyan on something that does not yet exist, the vow is valid because her hands do exist.
3. Rav Huna: A person can make a Kinyan on something that does not yet exist. Rav Nachman: The seller may retract even after the fruit has sprouted, because a person cannot make a Kinyan on something that does not yet exist. Rav Nachman agrees, however, that if the buyer seizes the fruit, we may not take it away from him.
4. Rebb Meir: It is a valid Kidushin, because a person can acquire something that does not yet exist. The same dispute applies in a case in which the man stipulates that the Kidushin should take effect after he or she is freed from servitude, or after her husband or sister dies, or after she does Chalitzah with her Yavam.
5. The Terumah is valid if he says that it should go into effect as soon as it reaches a third of its full growth, and it is subsequently picked from ground.
MA'ASROS FOR SOMETHING THAT DOES NOT EXIST
Rebbi Chiya derives from a verse that we may be lenient regarding Terumah and Ma'aser for the purpose of Kavod Shabbos and Yom Tov. Rashi explains that the leniency is that we may separate Ma'aser for something that we have not yet acquired. Tosfos asks why this is permitted only on Shabbos and Yom Tov. Since Rebbi Chiya maintains that we can make a Kinyan on something that does not yet exist, it should be permitted during the week as well. The Ritva answers that although one may separate Ma'aser for something that he does not yet own, it is a Mitzvah to separate Ma'aser from produce that has already been acquired. However, for the purpose of Kavod Shabbos, one may separate the Ma'aser from produce that he has not yet acquired.
A KINYAN ON SOMETHING THAT DOES NOT YET EXIST
A person cannot make a Kinyan on something that does not yet exist, both regarding a purchase and a gift. For example, if one says, "The produce that this field will produce shall be yours," or, "The offspring that my animal will produce shall be yours," the Kinyan is not valid even if the animal is pregnant. However, regarding the produce, the Kinyan is valid if it has already sprouted. (Shulchan Aruch CM 209:4)
If the fruit has sprouted, the Kinyan is valid, because it has come into existence. In contrast, the pregnancy of the animal is not regarded as though the offspring has come into existence, since it is hidden in the mother's stomach. (S'ma)
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