Nedarim, Chapter 4, Mishnah 6. The Mishnah deals with someone who forbade his fellow from deriving benefit from his property: Shimon forbade Reuven from benefiting from his property. Reuven may not borrow utensils from Shimon, since he is forbidden to derive benefit from him. And not only that, but he may not even lend utensils to Shimon, even though Shimon himself is not forbidden to benefit from Reuven. This is a rabbinic decree: if Shimon were permitted to use Reuven's utensils, Reuven might come to borrow utensils from Shimon as well. Therefore both sides were forbidden.
This principle applies to every form of business dealing:
Lending money: Reuven may not borrow money from Shimon, since he would be benefiting from him. And he may not lend money to Shimon either, lest as a result Shimon lend money to Reuven, which is forbidden.
Buying and selling: Reuven may not sell anything to Shimon, nor buy anything from him. Even selling to him for less than its value is forbidden, lest he come to buy from him for less than its value as well, and thereby benefit from him.
The second part of the Mishnah - the intent of the one who vows:
From here the Mishnah moves on to a vow of a different kind, one that does not involve two people who vowed against each other, but rather the clarification of the intent of the one who made the vow.
A man asked his fellow: lend me your cow, I need it. His fellow answered that the cow was not available, since it was busy working. The one who asked became angry that the favor had not been done for him, whether his fellow did not wish to do it or was simply unable to, and he vowed that his field would never be plowed by this cow.
Later on his fellow came and informed him that the cow was now available, and the one who made the vow now wishes to use it - except that he already vowed never to use it. The law depends on the circumstances and on his usual manner of conduct:
"Hayah darko lacharosh" - if he is accustomed to doing the plowing himself, then he is forbidden to use his fellow's cow, since the intent of his vow was that he himself would not plow with it. But any other person may plow the field of the one who vowed with this cow, because he intended only that he personally would not plow with it, not that plowing his field by others should be forbidden.
"Ein darko lacharosh" - if he is not accustomed to plowing himself but rather hires people for the work, then "hu v'chol adam asurin" - his intent was that his field should not be plowed by this cow at all, and therefore both he and any other person are forbidden to plow his field with it.
In summary: in this Mishnah we learned two matters. The first - one who is forbidden by vow to benefit from his fellow may not only not borrow, take a loan from, or buy from him, but may also not lend to him, extend him a loan, or sell to him, a decree lest he come to benefit from him. The second - the scope of a vow is determined by the intent of the one who vows: one whose practice is to plow himself forbade only himself; and one whose practice is not to plow himself forbade his field from being plowed by this cow by anyone at all.