2,789. When a Condition Changes or Is Mistaken

Hilchos Nedarim 8:2

Pursuant to the previous halacha (about conditions), if one made an oath or a vow not to marry a certain woman because her father is evil or not to enter a certain house because there’s a dangerous dog in it, then if the father or the dog died, or if the father repented, he may do the thing he vowed not to do. He basically said that he wouldn’t marry the woman or enter the unless the detriment was removed, which it now has been. The same is true in all comparable cases.

Hilchos Nedarim 8:3

However, let’s say that someone made a vow or an oath not to marry woman X because she’s unattractive and it turns out that she’s beautiful, or woman Y because she’s dark and it turns out that she’s fair, or woman Z because she’s short and it turns out that she’s tall, or someone takes a vow prohibiting his wife to benefit from him because she took his money and beat their son but it turns out that she didn’t do these things. In these cases, he is permitted because the vow was made in error, which is one of the vows whose prohibitions remain permitted. This is not the same as a vow that was taken conditionally and whose condition was not met because the reason for these vows never applied in the first place, being completely in error.