2,790. Unknown Circumstances
Hilchos Nedarim 8:4
Let’s take the inadvertent vows of the previous halacha one step further. Let’s say that someone saw from a distance that some people were eating his figs, causing him to declare them like a sacrifice to them. When he drew closer, however, he saw that it was his father and his brothers. In such a case, they may continue to eat the figs because, even though he didn’t express the reason for his vow, it’s understood that he only prohibited his produce to them because he thought they were strangers. The same is true in all comparable cases.
Hilchos Nedarim 8:5
If someone made a vow or an oath and then something came up that he hadn’t considered when he made it, what he prohibited remains forbidden until he asks a Torah scholar to absolve him of his vow. For example, if someone prohibited himself to benefit from a certain person or to enter a certain place, and then that person was made the local scribe or a shul was built in that place. Even though he wouldn’t have made the vow or the oath had he known that these things would happen, he may not benefit from that person or enter that place until he has his vow permitted, as has been discussed. The same rule applies in all similar situations.