He swore that something had happened on the basis of what he thought was most likely, then it turned out to be different than what he thought. What must he do?
Question

I had a phone call from one of my friends, and when we spoke I thought that it was not his voice, and I swore an oath stating: If you are my friend, I will give you one thousand dinars. Then I realized that it was indeed him. Right now I cannot give him that amount of money, because I owe a debt; I am not working and the amount is too great, even if I can manage to get the money. What should I do?
Praise be to Allah.If someone swears an oath about something, thinking it most likely that he is right, then it turns out to be different than what he swore the oath about, he does not have to do anything. This comes under the heading of unintentional oaths that Allah, may He be Exalted, has pardoned, according to the majority of the imams (Abu Hanifah, Malik and Ahmad).
Allah, may He be Exalted, says (interpretation of the meaning):
{Allah does not impose blame upon you for what is unintentional in your oaths, but He imposes blame upon you for what your hearts have earned. And Allah is Forgiving and Forbearing}
[al-Baqarah 2:225].
Al-Khiraqi (may Allah have mercy on him) said: Whoever swears an oath about something, thinking that it is as he attested it to be, then it turns out not to be like that, no expiation is required from him, because it comes under the heading of unintentional oaths. End quote.
Ibn Qudamah (may Allah have mercy on him) said: Most of the scholars are of the view that no expiation is required for such an oath. This was stated by Ibn al-Mundhir.
End quote from al-Mughni, 13/451.
Shaykh Muhammad al-Amin ash-Shinqiti (may Allah have mercy on him) said in Adwa’ al-Bayan, 1/447: There are several scholarly views as to what is meant by the unintentional oath in this verse.
The first view is that the unintentional oath is that which a man says in conversation, namely No by Allah, and Yes by Allah, without meaning it.
This view was favoured by ash-Shafa`i, and `A’ishah (may Allah be pleased with her) according to one of the two reports narrated from her. It was also narrated from Ibn `Umar, and from Ibn `Abbas in one of his two views.
The second view is that the unintentional oath refers to when a man swears on the basis of what he believes, then it turns out to be not true. This is the view of Malik ibn Anas, who said that it is the best that he had heard about the meaning of the unintentional oath. It was also narrated from `A’ishah and Abu Hurayrah, and from Ibn `Abbas in one of his two views.
These two views are similar, and the unintentional oath may refer to both of them, because in the first case he did not mean to swear an oath at all, and in the second case he only meant it to affirm what he thought was right and true. In linguistic terms, laghw (translated here as unintentional oath) refers to idle speech in which there is nothing good and that is unnecessary. An example of that is the hadith: “If you say to your companion, when the imam is preaching on Friday, ‘Be quiet,’ you have engaged in laghw.” End quote.
Shaykh Ibn Baz (may Allah have mercy on him) said:
The unintentional oath is an oath which a person does not intend in his heart and does not mean it as such; rather he says in conversation No by Allah, and Yes by Allah, by way of habit whilst talking, without intending it as an oath. What is meant by it being unintentional is that he does not mean it to be binding. So this oath is unintentional. Similar to that is when a person swears regarding something that he thinks most likely to be the case, then it becomes clear that he was incorrect, such as if he says: By Allah, I saw So and So, then it turns out that it was someone who looks like him, but it was not So and so, but he thought that he was right. This comes under the heading of unintentional oaths.
End quote from Fatawa Nur `ala ad-Darb, 24/237.
Based on that:
No expiation is required for this oath, and you do not have to give your friend one thousand dinars, because you swore this oath when you thought that you were right.
The Muslim should guard his oaths and not swear by Allah, may He be Exalted, except with regard to serious matters that deserve to be affirmed with an oath sworn by Allah.
And Allah knows best.