This verse: {Allah does not impose blame upon you for what is unintentional in your oaths} [Sūrat al-Baqarah: 225] was revealed concerning saying: "No, by Allah" and "Yes, by Allah"

Allah, the Almighty, says what means: {Allah does not impose blame upon you for what is unintentional in your oaths} [Sūrat al-Baqarah: 225]. According to ‘Ā'ishah (may Allah be pleased with her) the unintentional oath in this verse means what people usually say during their conversations with no intention of making an oath such as: "No, by Allah" and "Yes, by Allah". This ruling also applies to oaths that people make concerning past events, based on wrong assumptions.
Similarly, anyone who makes an oath believing that he is right, but then finds out that it is otherwise, does not have to offer expiation because it is an invalid oath according to the verse: {Allah does not impose blame upon you for what is unintentional in your oaths} [Sūrat al-Baqarah: 225]. According to this verse, Allah, the Almighty, will not punish people or make it binding on them to offer expiation for such unintentional oaths.
The same ruling applies to making an oath that someone would do something, believing that he is going to honor that oath, but he does not. Of course, this ruling applies with priority to the example mentioned by ‘Ā'ishah (may Allah be pleased with her) as she was present at the time of the revelation, and she was also knowledgeable about the language of the Arabs.