Praise be to Allah.
If someone who is fasting in Ramadan breaks his fast by deliberately having intercourse during the day, then he must offer expiation.
It was narrated that Abu Hurayrah (may Allah be pleased with him) said: Whilst we were sitting with the Prophet (blessings and peace of Allah be upon him), a man came to him and said: O Messenger of Allah, I am doomed!
He said: “What is the matter with you?”
He said: I had intercourse with my wife whilst I was fasting.
The Messenger of Allah (blessings and peace of Allah be upon him) said: “Do you have the means to free a slave?”
He said: No.
He said: “Are you able to fast for two consecutive months?”
He said: No
He said: “Do you have the means to feed sixty poor persons?”
He said: No.
The Prophet (blessings and peace of Allah be upon him) waited for a while, and whilst we were like that, a large basket of dates was brought to the Prophet (blessings and peace of Allah be upon him). He said: Where is the one who was asking?
He said: I am here.
He said: Take this, and give it in charity…
Narrated by al-Bukhaari (1936) and Muslim (1111).
So expiation by fasting is to be done by fasting for two consecutive months.
The way the two months are to be worked out is by using the lunar calendar, because in Islamic teachings the word month refers to the lunar month.
Allah, may He be exalted, says (interpretation of the meaning):
“They ask you, [O Muhammad], about the new moons. Say, ‘They are measurements of time for the people and for hajj”
That is, Allah, may He be exalted, by His grace and mercy, has caused them to change in this manner. The moon appears weak at the beginning of the month, then it gradually increases until halfway through the month, then it begins to decrease until the end of the month. This is so that the people will know thereby times for their acts of worship, such as fasting, as well as the times for paying zakaah and offering expiation, and the time of Hajj.
End quote from Tafseer as-Sa‘di (p. 88).
The lunar month may be twenty-nine days, or it may be thirty days, but no more than that. So the expiation of fasting for two consecutive months is not always sixty days; rather it may be fifty-eight days, or it may be fifty-nine days, and at most it will be sixty days.
Shaykh Ibn ‘Uthaymeen (may Allah have mercy on him) said:
With regard to the words “fasting for two consecutive months”, does this refer to the actual number of days in a lunar month (whether it is twenty-nine or thirty), or is it referring to a lunar month when it is complete (thirty days)?
Concerning this there are two scholarly views; the correct view is that what is referred to is lunar months, regardless of whether the month is complete (thirty days) or incomplete (twenty-nine days). So if someone asks: what is the difference between the two views?
The answer is that this may be explained by giving examples. If a person starts to fast these two months starting from the first night on which the new month is proven to have begun, let us say that it is the month of Jumaada al-Oola, and he begins fasting on the first day of the month, and concludes it on the last day of Jumaada al-Aakhirah, and let us assume that Jumaada al-Oola is twenty-nine days, and Jumaada al-Aakhirah is the same, then he will have fasted for fifty-eight days, and undoubtedly this is connected to the actual lunar months…
End quote from ash-Sharh al-Mumti‘ (6/413-414).
Based on this, if the one who is obliged to offer this expiation fasts for sixty days, then he will definitely have fasted for two months, and his expiation will undoubtedly be valid.
Ibn al-Mundhir (may Allah have mercy on him) said:
All the scholars from whom we learned are unanimously agreed that if a person fasts and does not count it according to the lunar months, if he fasts for sixty days, that is acceptable.
End quote from al-Ishraaf (5/308),
Conclusion: his expiation is valid, and there is no problem with it.
And Allah knows best.