He promised Allaah to pay charity if he did somenthing believing that it was a sin
15-3-2014 | IslamWeb
Question:
Assalaam u alaikum wa rahmat Ullah e wa brakatohu. A person is in the habit of performing some act which he believes to be a sin. He wants to abstain from the act so he promises Allah Subhanahu wa Ta'ala that he'll give a certain amount as charity if he performs the act. Later he learns that the act is actually permissible and NOT a sin. So the original assumption, on the basis of which the promise had been made, is now invalid. What are the options for him now? Does he have to offer the expiation for breaking the promise (feeding ten needy people etc.) if he wishes to perform the act? If yes, then will it be required just once or for every occurance of the act? Jazak Allah khair.
Answer:
All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad 
is His slave and Messenger.
The jurists stated that a promise that is ascribed to Allaah has the same legal status as that of an oath. Ibn Qudaamah 
said: “If a person says, 'I make a pledge and covenant with Allaah to do something, or 'By the pledge and covenant of Allaah, I will not do something, then this is an oath.” [Ash-Sharh Al-Kabeer]
The case in the question is an oath (Yameen) and a vow (Nathr) as it involves an offering to Allaah, and it is also a vow of obstinacy (Nathr Lajaaj) as it is meant to prevent oneself [from doing something] and is not meant to commit oneself to do something by which one gets closer to Allaah.
In any case, since what led you to make the oath is believing that such a matter is forbidden, and the reality proved otherwise, then there is nothing required of you, as the jurists stated that the reason that lead one to make the oath should be taken into consideration.
Ibn ‘Abdul Barr said in Al-Kaafi: “The basic principle in this issue is to take into consideration the intention of the person who made the oath, and if he had no intention, then we have to look at the reason that led him to make the oath and its effect on the oath, otherwise a ruling will be issued based on what is most likely according to the tendency of the people of his time.”
Ibn Al-Qayyim said in I’laam Al-Muwaqqi’een after he mentioned the statements of the scholars and cited quotes about taking into considration the reason that leads a person to make an oath: “What is meant is that the intention affects the oath - it either particularizes or generalizes it, and the reason is considered instead when there is no intention. Thus, the intention is inferred from and has the same effect as the cause [for the oath], and this is the Fatwa that must be issued in this regard.”
Therefore, if someone makes an oath to leave something believing that it is a sin, then it is permissible for him to do it if it becomes clear that that thing is permissible, and he does not have to atone for breaking the oath because he had made the oath for a reason that was proven afterwards to be non-existent. For more benefit, please refer to Fataawa 102811 and 86123.
Allaah Knows best.