The Sages permitted four nedarim: bargaining nedarim, hyperbolic nedarim, unwitting nedarim, and nedarim on conditions unavoidably unfulfilled.  What are bargaining nedarim? One was selling an item, and said: “Konam, if I sell to you for less than a sela,” and the other says: “Konam, if I pay you more than a shekel” —- both [persons] are willing at three dinars. R' Eliezer ben Yaakov says: So, too, one who wants to direct a neder at another so that the other person should eat with him.  He says: “Any neder that I will make in the future is void.” [This is effective] provided that he remembers at the time of the neder.