Is it permissible to delay fasting in expiation for breaking a oath until the heatwave is over?
Question

I want to ask: is it permissible to delay a fast in expiation for breaking an oath (kaffarat yamin) because of extreme heat? I only want to delay it until the heatwave is over, because I owe twelve days.
Praise be to Allah.Firstly:
The expiation for breaking a vow (kaffarat yamin) is mentioned by Allah, may He be exalted, 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. But guard your oaths. Thus does Allah make clear to you His verses that you may be grateful} [Al-Ma’idah 5:89].
So the one who broke his oath has the choice between three things: either feeding ten poor persons, or clothing them, or freeing a slave.
If he is unable to do any of these three, then he moves to the option of fasting, but it is not permissible for him to move to the option of fasting if he is able to feed ten poor persons.
For more information on the expiation for breaking an oath (kaffarat yamin), please see the answer to question no. 45676 .
Secondly:
If a Muslim breaks his vow, he must hasten to offer expiation for that immediately, and it is not permissible for him to delay it, so long as he is able to do it immediately.
Shaykh al-Islam Ibn Taymiyah (may Allah have mercy on him) said: Fulfilling a vow or offering expiation for breaking it, in our view, is to be done immediately."(Al-Fatawa al-Kubra 5/518).
Shaykh Ibn ‘Uthaymin (may Allah have mercy on him) said: Similarly, part of guarding oaths is offering expiation after breaking an oath. That expiation is required immediately, because the basic principle is that obligatory duties are to be done immediately, and that means what is required in the case of an oath."(Al-Qawl al-Mufid ‘ala Kitab al-Tawhid (2/456). See also: al-Sharh al-Mumti‘  15/159).
Thirdly:
If the Muslim is going to offer expiation for breaking his oath by fasting, and the weather is extremely hot, to the point that it makes fasting unusually difficult, then what appears to be the case is that it is permissible for him to delay fasting until the heatwave ends, so that his fast will be bearable. That is because it is permissible to delay doing obligatory duties, if there is a need or valid reason for doing so.
That is supported by the fact that the issue of whether expiation is to be offered immediately or that it may be done at any time is one of the issues concerning which differences of scholarly opinion are valid.
It says in al-Mawsu‘ah al-Fiqhiyyah (10/14): The majority of scholars are of the view that it is not permissible to delay offering expiation for breaking an oath, and that it must be offered as soon as the oath is broken, because this is what we should understand when the command is given without specifying a time. The Shafa‘is are of the view that offering expiation for breaking an oath is obligatory, but the timing for it is flexible. End quote.
As following the view that it must be done immediately will be extremely difficult for the accountable person, it is permissible for him to follow the view that offering expiation is flexible and may be delayed.
Moreover, the scholars have stated that it is permissible to delay giving zakah, if there is a valid reason or need to do so, and the obligation to give zakah is more important than the obligation to offer expiation for breaking an oath, because it is one of the pillars of Islam.
The scholars have stated that a person may delay giving zakah if he is poor and needs to keep this money, then he may give it when his situation improves, and he may delay it in order to give it to a relative or to one who is in greater need.
See: al-Insaf by al-Mawardi (7/141).
As it is permissible to delay giving zakah because of need or because there is some purpose to be served by delaying it, the same may be said with regard to the expiation for breaking an oath, because its obligation is less important than the obligation of giving zakah.
And Allah knows best.