Nedarim 9:2-3

Nedarim 9:2

Rabbi Eliezer said that they can find an opening to cancel a vow based on new circumstances, though the Sages do not allow this. For example, if one said, “Konam that I do not benefit from such-and-such person” and that person later became a scribe, or that person is going to make a wedding soon, so the one who vows says, “Had I known that he was going to become a scribe or make a wedding, I wouldn’t have vowed.” Another example is if he says, “Konam that I enter this building” and it became a shul, so he says, “Had I known that it would become a shul, I never would have vowed.” In such cases Rabbi Eliezer permits canceling the vow and the Sages prohibit it.

Nedarim 9:3

Rabbi Meir says that some things are like new situations even though they really aren’t; the Sages disagree. For example, if a person says, “Konam I do not marry such-and-such girl, whose father is evil” and he is informed that that person died or repented from his evil ways. Or if he says, “Konam that I enter this house because there is a vicious dog in it” or “a snake in it” and he is informed that the dog died or the snake was killed. These are like new situations even though they really aren’t. The Sages disagree in this matter.