Must the monetary expiation of an oath, paid before learning of its invalidity, be repaid, given the ignorance of its number?

The issue of giving money as expiation for an oath (kaffarat al-yameen) is a matter of ijtihad (independent reasoning) about which scholars have differed. The majority of them do not permit it, while Abu Hanifa allowed it. In matters of ijtihad, there is no blame on someone who follows a scholar, and what they have done is valid and sufficient. If they later discover a more authoritative opinion, they should adopt it, but what has passed is sufficient and they are not commanded to repeat it. This is a general principle in matters of ijtihad. Sheikh al-Islam Ibn Taymiyyah stated that these matters should not be condemned by force (bi’l-yad), and there is no condemnation against someone who follows an opinion in them. Moreover, any wealth a person acquires through transactions about which there is scholarly disagreement, due to an interpretation or following a scholar, that wealth belongs to them, and they are not required to disburse it even if they later realize its error. However, they must repent and abandon such transactions. Sheikh Ibn Uthaymeen was asked about giving zakat al-fitr in cash, and he replied that it is not sufficient, except for someone who followed those who permitted it and was unaware of the correct ruling.