What is the ruling on swearing an oath and then forgetting it, and is expiation for the oath obligatory in this case and deferred due to inability to pay it, and what is the ruling on swearing an unintentional oath?

A Muslim should not swear by Allah Almighty excessively, as this is legally reprehensible. Expiation (kaffarah) is obligatory if a deliberate oath is broken. If the number of oaths is unknown, one should offer expiation for what is most likely to clear their conscience. Expiation consists of feeding ten needy persons, or clothing them, or emancipating a slave. If one cannot find any of these, then fasting for three days. If one is unable to fast, it remains a debt upon them. As for oaths uttered unintentionally, they are considered vain oaths (laghw al-yameen) for which there is no expiation.