1) A NEDER WHICH PROHIBITS FRUIT THAT GROWS DURING SHEMITAH
QUESTION: The Mishnah states that when one makes a Neder before the Shemitah year in which he declares that his friend should not have any pleasure from his possessions, his friend is prohibited from eating his fruits during Shemitah even though they are Hefker and do not belong to the original owner (the one who made the Neder).
The Gemara explains that this law applies only when he said "these possessions are prohibited to you." If he said "my possessions are prohibited to you," his friend may eat the fruit during Shemitah since the fruit no longer belongs to the original owner.
Although one may make a Neder to prohibit his friend from having pleasure from his possessions, one may not make a Neder to prohibit his friend from possessions that he does not own (see 57a). Why, then, does the Neder which he makes before Shemitah to prohibit his friend from eating his fruit during Shemitah take effect? At the time of the Neder the fruit does not exist, and when the fruit comes into existence during Shemitah it is Hefker and does not belong to him!
(The Mishnah cannot mean that he makes the Neder prior to Shemitah when his trees are already bearing fruit which will be picked only during Shemitah, because fruit which began to grow before Shemitah is not considered fruit of Shemitah and does not become Hefker.)
ANSWER: In the Neder, the person did not explicitly prohibit his fruit to his friend. Rather, he made a Neder to prohibit his possessions to his friend. Since his trees are part of his possessions, they are prohibited to his friend, and therefore the fruit which grows from those trees is also prohibited. (See KEHILOS YAKOV who discusses this issue at length.)
2) HALACHAH: FRUITS AND FIELDS DURING SHEMITAH
The Gemara asks why a Mudar Hana'ah is permitted to eat fruit from his friend's field during the Shemitah year, but he is not permitted to enter the field and walk on the land. Both the fruit and the land are Hefker during Shemitah! The Gemara answers that he is not permitted to enter the land because he might remain on the land even after he finishes picking the fruit.
This Gemara has important implications for the Halachos of Shemitah.
(a) It is clear from the Gemara that one's land is not entirely Hefker during the Shemitah year. The owner is required only to treat it like Hefker by letting others come in to pick the fruit. The owner does not have to let others enter the land for other purposes.
(b) The Acharonim infer from the Gemara the answer to the question of whether a landowner must make his field and fruit Hefker during Shemitah, or whether it automatically becomes Hefker once the Shemitah year arrives.
1. The RAN's text of the Gemara reads "Rachmana Afkerei," which implies that the Torah automatically makes one's fruit Hefker and makes the use of one's field Hefker for the sake of picking the fruit. The owner does not need to make them Hefker himself. The Mitzvah requires only that the owner treat them as Hefker and not prevent others from entering and picking the fruit. RASHI, TOSFOS, and the ROSH also have the Girsa, "Rachmana Afkerei." The MAHARIT (#43) proves from here that the Torah automatically makes the land and fruit Hefker during Shemitah. (The Maharit who discusses this issue at length.)
2. The PE'AS HA'SHULCHAN (Hilchos Shevi'is 23:29) disagrees and says that since the Girsa of our texts is "Ar'a Nami Afkerei" -- "the land is also Hefker," there is no implication that the owner does not need to declare his land Hefker.
The Maharit maintains that the status of Hefker of Shemitah automatically takes effect on all land in Eretz Yisrael, even landed owned by a Nochri. Accordingly, there is no obligation to separate Terumos and Ma'aseros from fruit grown by Nochrim in Eretz Yisrael during the Shemitah year (since Hefker is exempt from Terumos and Ma'aseros; however, there is an obligation to separate Terumos and Ma'aseros from the produce of a Nochri during all other years, if a Jew performed the Gemar Melachah). In contrast, the Pe'as ha'Shulchan maintains that one should separate Terumah from such produce.
The issue of separating Terumos and Ma'aseros from such fruit is directly related to the well-known dispute between the BEIS YOSEF and the MABIT about whether the laws of Shemitah apply to produce grown on land owned by Nochrim. If the laws of Shemitah apply to such fruit, there is no requirement to separate Terumos from them. The Beis Yosef and the Pe'as ha'Shulchan rule that such fruits do not have Kedushas Shevi'is, and thus Terumos and Ma'aseros must be separated from them. This was the generally accepted custom of the old Yishuv in Yerushalayim.
The Mabit, the Maharit (the son of the Mabit), the Sefer ha'Charedim, the Shelah, and the Chayei Adam disagree. The CHAZON ISH, too, was a strong proponent of this opinion (that produce grown on land owned by Nochrim in Eretz Yisrael has Kedushas Shevi'is) and, as a result, many have adopted this as the accepted practice.