The scholars differed concerning the issue of giving money for the expiation for breaking an oath. There are two opinions: 

1.

The view of the majority of Maaliki, Shaafa’i and Hanbali scholars is that it is not acceptable to give money instead of food, adhering solely to the text concerning that, in which Allaah says (interpretation of the meaning):  

“Allaah will not punish you for what is unintentional in your oaths, but He will punish you for your deliberate oaths; for its expiation (a deliberate oath) 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. But whosoever cannot afford (that), then he should fast for three days. That is the expiation for the oaths when you have sworn. And protect your oaths (i.e. do not swear much). Thus Allaah makes clear to you His Ayaat (proofs, evidences, verses, lessons, signs, revelations, etc.) that you may be grateful”

[al-Maa’idah 5:89]

It says in Mawaahib al-Jaleel (3/272), which is a Maaliki book: 

The cash value is not acceptable instead of giving food or clothing. End quote. 

Ibn Qudaamah al-Hanbali (may Allaah have mercy on him) said: 

In the case of expiation, it is not acceptable to give the value of the food or clothing, according to the view of our imam, and Maalik, al-Shaafa’i and Ibn al-Mundhir. This is also the apparent meaning of the statements of ‘Umar ibn al-Khattaab, Ibn ‘Abbaas, ‘Ata’, Mujaahid, Sa’eed ibn Jubayr and al-Nakha’i. 

This is indicated by the following: 

i.The words of Allaah (interpretation of the meaning): “feed ten Masaakeen (poor persons), on a scale of the average of that with which you feed your own families”. This clearly refers to actual food or clothing, so expiation is not achieved by anything else, because it does not fulfil the obligation, as it does not fulfil what Allaah has enjoined.

ii.Because Allaah has given the choice of three things. If paying the (monetary) value were permissible, the choice would not have been limited to three things.

iii.Because if the (monetary) value was intended, there would be no point in offering the choice, because if the value of food was the same as the value of clothing, they would be the same thing, so how could one choose between them? And if the value of one of them was higher than the other, how could one choose between one thing and part of it?

iv.Moreover, if he gives him money to clothe himself and what he gives is equal to what he is supposed to give for food, if we accept the view that the expiation may be given in the form of money, that should be acceptable, but it is contrary to the aayah. The same applies if the cost of food rises so that half a mudd would cost the same as clothing a poor person;  if we accept the view that the expiation may be given in the form of money, then half a mudd should be acceptable, but it is contrary to the aayah.

v.Because what is mentioned is specific things, giving the equivalent value in cash is not sufficient. Therefore, if he gives them many times more than the value of the food, it does not count.

End quote from al-Mughni (11/257). 

2.

The second view is the Hanafi view, which is that it is acceptable to offer money as expiation. 

Al-Sarkhasi [al-Hanafi] (may Allaah have mercy on him) said: 

Paying the monetary value instead of what is mentioned in the texts in the case of zakaah and expiation is permissible in our view. 

The more correct of the two opinions is the first one, which is that it is not acceptable to pay the monetary value in the case of expiation for breaking an oath (kafarat yameen) and other kinds of expiation. 

See also the answer to question no. 20881 and 45676. 

And Allaah knows best.