Nedarim, Chapter Four, Mishnah Six
The first section continues to discuss prohibitions applicable to a person who cannot derive benefit from another because of a vow. The second section is a digression from the topic of our chapter.
1) He who is forbidden by vow to benefit from his neighbor may not lend [objects] to him or borrow from him, lend [money] to him or receive from him a loan, sell to him or purchase from him.
2) One says to another, Lend me your cow. [The other] says, It is not available. [The first one] says, Konam, if I ever plow my field with it.
a) If he generally plowed himself, he is forbidden, but others are permitted.
b) But if he did not generally plow himself, he and others are forbidden.
Section one: If Reuven cannot receive benefit from Shimon, he certainly may not borrow from him. Our mishnah teaches that he is prohibited even from lending to him, lest by doing so he also borrows. This fear of reciprocity is the reasoning that lies behind the following mutual prohibition as well. Obviously, Reuven cannot borrow money, but the mishnah teaches he may not even lend money to Shimon, lest he borrow as well. Reuven cannot sell or buy lest he benefit from either transaction.
Section two: This section is not directly related to the larger topic of this chapter. It is brought here because it also has to do with lending and vowing.
Reuven asks Shimon to borrow his cow to plow with it. Shimon responds that his cow is not available. Reuven, being somewhat impatient, takes a vow that he will never use Shimons cow to plow his field. The mishnah rules that if Reuven normally plows on his own, Shimon can lend him the cow and others can plow Reuvens field with it. In other words, we interpret Reuvens vow literally; Reuven cannot plow with it, but others can. However, if Reuven does not normally plow, then obviously when he said Konam, if I ever plow my field with it, then he meant Konam, if my field is ever plowed with it. Therefore, even others may not plow his field with Shimons cow.