2,783. One Who Prohibits Himself Benefit From an Entire Nation

Hilchos Nedarim 7:7

Let’s say that someone prohibited himself from benefiting from a particular nation. In such a case, he may buy from them at above the market price and he may sell to them at below the market price. If he prohibits them from benefiting from him, then he may buy from them at below the market price and sell to them at above market price, if they are so willing. We don’t prohibit him to sell to them as a preventive measure against buying from them because he didn’t vow about just one person, in which case such a strategy is appropriate. Rather, he vowed about an entire nation, so if he can’t do business with one person, he would just do business with someone else. Accordingly, if he prohibited himself to benefit from them, he may lend objects and money to them but he may not borrow from them.

Hilchos Nedarim 7:8

If he both prohibited them to benefit from him and vice versa, he may not do business with them at all. He may not borrow from or lend to them, neither objects nor money.