Firstly:
What is the expiation for breaking an oath (kaffaarat al-yameen)?
The options for expiation for breaking an oath are in accordance with the order in which Allah, may He be glorified and exalted, mentions them in Soorat al-Maa’idah, where He says (interpretation of the meaning):
{Allah will not impose blame upon you for what is meaningless in your oaths, but He will impose blame upon you for [breaking] what you intended of oaths. So its expiation is the feeding of ten needy people from the average of that which you feed your [own] families or clothing them or the freeing of a slave. But whoever cannot find [or afford it] – then a fast of three days [is required]. That is the expiation for oaths when you have sworn. But guard your oaths. Thus does Allah make clear to you His verses that you may be grateful} [al-Maa’idah 5:89].
The one who is offering this expiation must choose one of these three options and do it:
Feeding ten poor persons from the average of that which he feeds his [own] family,
Or clothing them,
Or freeing a slave.
The one who does one of these things has discharged his duty and has done what is required of him. If he is unable to do any of these three options, he moves to the option of fasting, and he must fast three days.
The amount of food that must be given
The amount of food that must be given is half of one saa‘ to each poor person, which is equivalent to approximately one and a half kilograms of rice and the like. If some kind of sauce is provided with it, that is better. It is sufficient to provide lunch or dinner to ten poor people. In terms of clothing, it is sufficient to give a chemise (thobe) to each poor person.
It is not permissible to move to the option of fasting when one is able to feed or clothe poor people or free a slave, because Allah, may He be exalted, says (interpretation of the meaning) {But whoever cannot find [or afford it] – then a fast of three days [is required]} [al-Maa’idah 5:89].
Ibn al-Mundhir (may Allah have mercy on him) said: The scholars are unanimously agreed that if the one who made an oath can afford to feed or clothe poor persons, or free a slave, it is not valid for him to fast if he breaks his oath." (Al-Ijmaa‘ p. 157).
See also the answer to question no. 178167 .
Secondly:
Why did the Qur’an say {from the average of that which you feed your [own] families} with regard to the expiation for breaking an oath?
The words {from the average of that which you feed your [own] families} do not mean that the one who wants to offer expiation in this way must be one who is feeding his family; rather this is by way of clarifying that the food should be the average type of food that he or his family eat.
Because the individual is the one who will offer expiation, it is said to him: If you were the one who is responsible for feeding your family, what would you give them? The answer is what he should give.
Perhaps– and Allah knows best – there is a subtle meaning in the usage of the phrase “you feed” instead of “you eat”, and what is meant here is to highlight that the one who is offering expiation has a choice regarding what kind of food he gives. The one who eats from what someone else gives him only eats what is given to him by the one who is spending on his maintenance, so usually he cannot choose for himself, and it is not appropriate for that scenario to be an analogy when it comes to feeding the poor. On the other hand, the one who spends on his family is the one who chooses high-quality or low-quality food, because he is in charge of spending, so the verse explains to him what he should give to the poor. He has the option of feeding his family high-quality food sometimes and low-quality food at other times, so let him choose a middle path between the two. Thus he is not required to give the poor the highest-quality food that he sometimes gives to his family when he has the means, and he should not give the lowest-quality and poorest-quality food to them. Rather he should choose something in between the two.
Ibn al-‘Arabi (may Allah have mercy on him) said:
The words {you feed} [al-Maa’idah 5:89] may be understood as referring to what you feed your family in general, or it may be understood as meaning the average of what is eaten at lunch and dinner. The ummah is unanimously agreed that what is to be offered as expiation should be moderate in amount, and in cases other than offering expiation should be filling, but Abu Haneefah said: The amount to be offered as expiation for breaking an oath has been worked out as half a saa‘ in the case of wheat, and one saa‘ in the case of dates and barley.
To examine the matter further, we may note that the word wasat [the root of the word awsat, translated above as average] may refer to the best and finest in quality, as seen in the verse in which Allah, may He be exalted, says: {And thus We have made you a community that is wasat} [al-Baqarah 2:143], meaning that which is best in character. Or wasat may refer to something that is in the middle, halfway between two extremes. That is how we understand the proverb, “The best course is the middle course.”
The ummah is unanimously agreed that the meaning of wasat in the sense of the best quality is not what is intended in this verse, and they are agreed that what is meant is the middle between two extremes. Some of them said that it is to be worked out on the basis of custom, and some of them worked out the amount to be given, such as Abu Haneefah.
Some of the scholars erred when they said that if he only eats barley bread, and the people eat wheat bread, then he should give what the people eat.
This is an obvious mistake, because if the one who has to offer expiation for breaking an oath can only afford to eat barley bread, he should not be enjoined to give others something other than what he eats. The Prophet (blessings and peace of Allah be upon him) said: “A saa‘ of foodstuff, a saa‘ of barley, a saa‘ of dates,” in a place where barley and dates were more readily available than wheat. He only mentioned that so that each person could give to the poor what he eats himself. This is quite clear.
With regard to the expiation for shaving the head [due to an ailment of the scalp] when in ihram, the Prophet (blessings and peace of Allah be upon him) explained what was meant by the brief word “charity” [in al-Baqarah 2:196] by stating that it was a farq [unit of measure] to be distributed among the six poor persons; the farq is equivalent to three saa‘s. This is what was referred to in one brief word. But Allah, may He be glorified and exalted, did not refer to the expiation for breaking an oath by using one brief word; rather He said: {from the average of that which you feed your [own] families} [al-Maa’idah 5:89].
And Allah knows best.