Nedarim, chapter 4, mishnah. The mishnah before us deals with a person who has been forbidden to derive benefit from his fellow, at a time when the Sabbatical year removes the owner's rights to his produce. Four cases are presented here, divided according to two variables: whether the vow covered benefit in general or only eating, and whether it was made before the Sabbatical year or during it.
Where the vow forbade benefit and was made before the Sabbatical year:
Shimon vowed that Reuven may not derive benefit from him, and the vow was made before the Sabbatical year. During the Sabbatical year itself Reuven may not enter Shimon's field, since the field remains Shimon's property even in the Sabbatical year, and entering it is a benefit.
Not only that, but he may not even eat from his produce, and this includes produce that stretches out and hangs beyond the boundary of the field, where there is no trespassing and no benefit taken from the field itself. Even though in the Sabbatical year the produce becomes ownerless and is no longer in Shimon's possession, we have a general rule that a person can forbid to his fellow something that is in his possession, and the prohibition takes effect even after the item leaves his possession. Since the vow was made while the produce was still in Shimon's possession, Reuven is forbidden to derive benefit from it even after it became ownerless in the Sabbatical year.
Where the vow forbade benefit and was made during the Sabbatical year:
If Shimon declared during the Sabbatical year itself that Reuven may not derive benefit from him, here too Reuven does not enter the field. Strictly speaking there would have been room to permit it, for as will be explained in the next ruling he is allowed to eat from the produce of the field as long as he need not enter it, since at the time of the vow the produce was not in Shimon's possession; and entering the field in order to pick the ownerless produce would in itself be permitted. The concern, however, is that he may linger in the field longer than the time needed to pick the produce, and would then be deriving benefit from Shimon's field, and therefore they did not permit him to enter.
Where the vow forbade eating and was made before the Sabbatical year:
Shimon vowed that Reuven is not forbidden in general benefit but only in eating - that he may not eat from what belongs to him - and the vow was made before the Sabbatical year. In this case Reuven may go down into his field, since he is not forbidden to derive benefit, and entering the field is only a general benefit, which is permitted. But he may not eat from the produce even in the Sabbatical year, because of the very rule explained in the first clause: a person vows before the Sabbatical year and his vow takes effect even after the produce becomes ownerless, since he forbade it while it was still in his possession.
Where the vow forbade eating and was made during the Sabbatical year:
If Shimon vowed during the Sabbatical year that Reuven is forbidden only in eating and not in benefit, Reuven may enter Shimon's field, since entering presents no problem, and he may even eat from the produce, for at the time Shimon made his vow the produce did not belong to him and he had no power to forbid it to Reuven.
In summary: two principles underlie this mishnah. The first: a person can forbid to his fellow something that is in his possession, and the prohibition takes effect even after the item leaves his possession; therefore a vow made before the Sabbatical year forbids the produce even after it becomes ownerless, whereas a vow made during the Sabbatical year does not take effect on it at all. The second: the distinction between a vow forbidding benefit and a vow forbidding eating. A vow forbidding benefit forbids even entering the field (and where the vow was made during the Sabbatical year, out of concern that he may linger there longer than the time needed to pick the produce), whereas a vow forbidding eating does not forbid entering, since that is no more than a general benefit.