He vowed to break the fast in Ramadaan

Praise be to Allah.

Firstly: fasting Ramadaan is obligatory for adult, sane Muslims who are able to fast. If a person meets those criteria, it is haraam for him to break the fast with no excuse. It is also haraam for him to swear to do that, because by swearing such an oath he is resolving to do something haraam. 

Secondly: 

If a Muslim swears to commit a sin, it is not permissible for him to do what he swore to do, rather he must break his vow and offer expiation for breaking an oath (kafaarat yameen), because the Prophet (peace and blessings of Allaah be upon him) said: “Whoever vows to do something then sees that something else is better than it, let him do what is better and offer expiation for his vow.” Narrated by Muslim, 1650. 

Kafaarat yameen (expiation for breaking an oath) means feeding ten poor persons, or clothing them, or freeing a slave. If a person cannot do any of these things, then he must fast for three days, because 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]

See also question no. 45676

Thirdly: 

The one who has done that must repent to Allaah, for it is very repugnant for a Muslim to swear to break the fast in Ramadaan. This indicates that he is careless about the sacred limits set by Allaah, and that he takes them lightly. In the answer to question no. 38747 we stated that it is very dangerous to break the fast in Ramadaan with no excuse, and that the one who does that is regarded as a hypocrite, Allaah forbid. 

Al-Dhahabi said in al-Kabaa’ir (p. 64): 

Among the believers it is well established that the one who fails to fast in Ramadaan although he is not sick and he has no excuse that would permit that, is worse than an adulterer or one who is addicted to alcohol; indeed they would have doubts about his Islam and would think that he is a heretic and a deviant. End quote. 

We ask Allaah to keep us safe and sound and to make us steadfast in adhering to His religion. 

And Allaah knows best.