The Torah requires that once every seven years agricultural activity in the Land of Israel ceases. During this Sabbatical year, the produce that grows is considered ownerless and is readily available to all. The Mishna on our daf teaches that a person who makes a vow prior to the Sabbatical year that he will not derive benefit from his friend, cannot enter onto his property or eat fruit from his trees even if they are growing on branches that grow outside of his property. During the Sabbatical year itself, however, even though he cannot enter his friend’s property, he would be allowed to eat fruit from the trees since the fruits are hefker – they are considered ownerless. The Mishna continues by discussing a person whose vow related only to deriving benefit from eating. Generally speaking, such a person cannot take fruit, but can enter the property of the other person; on the Sabbatical year he can also eat fruit, since it is not owned by anyone.
There are two approaches to this Mishna.
The majority of the rishonim – the Rosh, Ritva and others – understand that our Mishna is not dealing with someone who makes a neder during the Sabbatical year, rather it is only talking about someone who makes the neder prior to the Sabbatical year. Thus, the first ruling that appears in the Mishna is obvious – a person cannot derive benefit from someone who he has forbidden by means of a vow. The point that is being made by the Mishna is that of the second case. During the Sabbatical year, even though entering the person’s property remains forbidden, the fruits are permitted.
Rashi and Tosafot suggest that the Mishna is discussing two different types of vows. In one case, the vow is taken prior to the Sabbatical year, and both entering the field and benefiting from the fruits is forbidden – even during the Sabbatical year, since by means of a neder a person can affect the status of an object even after it has left his possession. In the second case, the vow is taken during the Sabbatical year itself, and only entering the person’s property will become forbidden, since the vow cannot affect the fruits, which do not belong to him.