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.

[What are] hyperbolic nedarim? [A person] said: “Konam, unless I have seen on this road like those departing from Egypt,” [or] “unless I have seen a snake like a beam of the olive press.”  [What are] unwitting nedarim? [A person said:] “If I ate,” or “If I drank,” and he remembers that he ate or drank; “If I eat,” or “If I drink,” and he forgot, and ate or drank; [or a person said:] “Konam my wife benefiting from me, because she stole my purse,” or “she beat my son,” and it was discovered that she had not beaten him, and it was discovered that she had not robbed him.  A person saw people eating figs, and he said: “[They are] korban to you,” and it turned out to be his father and his brothers, and there were others with them —- Beis Shammai say: They are permitted, but those with them are forbidden; Beis Hillel, however, say: Both are permitted.