Is it valid as expiation for breaking an oath (kaffarat yameen) to buy food for the poor from a restaurant?
Question

Is it permissible to buy ready-made food from a restaurant and give it as expiation for breaking an oath, or is it like zakaah al-fitr and should be in the form of a bag of rice?
Praise be to Allah.Allah, may He be exalted, mentions the expiation for breaking an oath in the verse (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} [Al-Maa’idah 5:89].
So what you must do is choose one of the three options (feeding ten poor persons, clothing them, or freeing a slave). If the Muslim does any one of these three things, that is acceptable; if he is not able to do any of them, then he moves to the option of fasting.
These options have been explained in detail in the answer to question no. 45676 .
If the Muslim chooses the option of feeding poor persons, then he has the choice between the following:
He may either give the poor people uncooked food (such as a bag of rice), or he may give them cooked food, or he may make the food in his house and invite ten poor people to come and eat. All of these options are valid and acceptable, as has been stated by the scholars.
Ibn al-Qayyim (may Allah have mercy on him) said: What is indicated by the texts of the Quran and Sunnah is that what is obligatory in the case of expiation and maintenance is to provide food, and it is not stipulated that the recipient should be given food that he can take away with him. So if the poor person is invited to a meal, that is valid.
He said: This is what is proven from the Sahaabah (may Allah be pleased with them).
Then he narrated that this is what was done by ‘Ali, Ibn Mas‘ood, Ibn ‘Umar, and Abu Moosa al-Ash‘ari (may Allah be pleased with them).
He said: This is the view of the people of Madinah, the people of Iraq, and Ahmad, according to one of the two views narrated from him. Allah, may He be exalted, did not mention ta‘aam (food), which refers to what is eaten; rather He  mentioned it‘aam (feeding), which refers to giving food. This clearly indicates that if a person feeds poor people, and does not give them food to take away with them, then he has done what he was instructed to do, and it is true that he has fed them, both in linguistic terms and in terms of what is customary." (Zaad al-Ma‘aad  5/441-445).
The scholars of the Permanent Committee for Ifta’ said:
In that regard [i.e., offering expiation for breaking an oath] it is acceptable for you to provide lunch or dinner to ten poor persons, or to give each one of them half a saa‘ of wheat or rice, or any other kind of staple food that they usually eat. End quote.
Permanent Committee for Academic Research and Ifta’.
Shaykh ‘Abdullah ibn Qa‘ood, Shaykh ‘Abdullah ibn Ghadyaan, Shaykh ‘Abd ar-Razzaaq ‘Afeefi, Shaykh ‘Abd al-‘Azeez ibn ‘Abdillah ibn Baz.
Fatawa al-Lajnah ad-Daa’imah (23/10).
They were asked about expiation for breaking an oath: is it obligatory to give ready-made, cooked food, or is it permissible to give dry food like rice and wheat?
They replied: Both are acceptable. So if a person makes food and invites ten poor people, or he gives five saa‘s, giving half a saa‘ to each poor person, that is acceptable."(Fataawa al-Lajnah ad-Daa’imah li’l-Buhooth al-‘Ilmiyyah wa’l-Ifta’)
Shaykh ‘Abdullah ibn Qa‘ood, Shaykh ‘Abdullah ibn Ghadyaan, Shaykh ‘Abd ar-Razzaaq ‘Afeefi, Shaykh ‘Abd al-‘Azeez ibn ‘Abdillah ibn Baz.
Fatawa al-Lajnah ad-Daa’imah (23/14).
They were asked about someone who owes expiation for breaking an oath, so he gave the owner of a restaurant fifty riyals so that he could feed ten poor persons, as the owner of the restaurant told him that feeding one person costs five riyals.
They replied:
The expiation is to feed ten poor persons, giving each poor person half a saa‘ of food, which is equivalent to one and a half kilograms; or to clothe ten poor persons, giving each  poor person a thobe; or freeing a believing slave. If he cannot afford to do any of these options, he must fast for three days.
If the owner of the restaurant whom the one who had broken his oath appointed as his proxy did what was needed, and fed ten poor persons, then that is acceptable, praise be to Allah.
End quote from Fataawa al-Lajnah ad-Daa’imah li’l-Buhooth al-‘Ilmiyyah wa’l-Ifta’.
Shaykh Bakr Abu Zayd, Shaykh ‘Abd al-‘Azeez Aal ash-Shaykh, Shaykh Saalih al-Fawzaan, Shaykh ‘Abdullah ibn Ghadyaan, Shaykh ‘Abd al-‘Azeez ibn ‘Abdillah ibn Baz.
Fatawa al-Lajnah ad-Daa’imah (23/121).
Thus it is clear that if you bought food from the restaurant and gave it to ten poor persons, then this is acceptable as expiation for breaking an oath.
And Allah knows best.