He offered Kaffarat al-Yamin in the form of cash; does he have to offer it again?

Praise be to Allah.

Offering Kaffarat al-Yamin in the form of cash is not valid according to the majority of Maaliki, Shaafa’i and Hanbali scholars. It is valid according to the Hanafis. 

Ibn Qudaamah al-Hanbali (may Allah have mercy on him) said: It is not valid to give the value of food or clothing as expiation according to the view of our imam [Ahmad], Maalik, al-Shaafa’i and Ibn al-Mundhir. This is also the view of ‘Umar ibn al-Khattaab, Ibn ‘Abbas, ‘Ata’, Mujaahid, Sa’eed ibn Jubayr and al-Nakha’i. 

Al-Awzaa’i and as-haab al-ra’i allowed it, because the purpose is to meet the need of the poor person, and that may be done by giving the equivalent value. 

What proves that this expiation should be done by giving food or clothing or freeing a slave, and that it is not valid to give the value in cash, is the verse in which Allah says (interpretation of the meaning): “…feed ten Masaakeen (poor persons), on a scale of the average of that with which you feed your own families, or clothe them or manumit a slave” [al-Maa’idah 5:89]. The apparent meaning points to actual food or clothing, so expiation cannot be achieved otherwise, because one is not doing what is required if one does not do it in the way that Allah instructed. And Allah has given us the choice between three things; if giving the value were permitted, the choice would not be limited to three. End quote from al-Mughni (10/6). 

Shaykh Ibn Baaz (may Allah have mercy on him) said: Kaffarat al-Yamin should take the form of food, not cash, because this is what is mentioned in the Holy Quran and the Sunnah. What must be given is half a saa’ of the local staple food, be it dates, wheat or anything else. This is equivalent to approximately one and a half kilograms. If you offer them lunch or dinner, or you clothe them in something that is sufficient to pray in, that is acceptable, namely a thobe or a lower garment and upper garment."(Fatawa Islamiyyah  3/481). 

But if you did this on the basis of the Hanafi opinion, or a fatwa given to you by someone who follows their view, then that is valid, otherwise you have to offer it again. That will be better for you and your poor brothers. 

And Allah knows best.