Answer: If he is among those permitted to break the fast—such as the sick person or the traveler—then there is no harm in doing so. But if he is not permitted to break the fast, he must abstain for the rest of the day and make up the fast, while bearing sin.
Our statement that he must make it up is because, once he began it, he made it binding upon himself, so it became, in his case, like a vow. This differs from one who deliberately did not fast at all from the outset: such a person is not to make it up; and even if he were to make it up, it would not be accepted from him, due to the Prophet’s saying: “Whoever performs an action that is not in accordance with our matter shall have it rejected.” (1)
As for the hadith, “Whoever deliberately breaks his fast for one day of Ramadan, the fasting of a lifetime will not make up for it,” (2) it is a weak hadith. Even assuming it to be authentic, its meaning is that it will not be equivalent to having fasted it at its proper time.
(1) Its authentication and source references have previously been given on p. 172.
(2) Al-Bukhari cited it in suspended form, using a tentative wording, from Abu Hurayrah, may Allah be pleased with him, as a marfu report (attributed to the Prophet), in The Book of Fasting, chapter: “If One Has Intercourse During Ramadan.” It was linked with a complete chain by Abu Dawud, The Book of Fasting, chapter: “The Severe Warning Regarding One Who Deliberately Breaks His Fast” (2396); al-Tirmidhi, The Book of Fasting, chapter: “What Has Been Reported Regarding Deliberately Breaking the Fast” (723); al-Nasa'i, Al-Sunan al-Kubra (3265), al-Risalah edition; and Ibn Majah, The Book of Fasting, chapter: “What Has Been Reported Regarding the Expiation for One Who Breaks a Day of Ramadan” (1672).
It was also narrated as a mawquf report (stopping at the Companion) from Ibn Masud, may Allah be pleased with him. Al-Bukhari cited it in suspended form in The Book of Fasting, chapter: “If One Has Intercourse During Ramadan,” and it was linked with a complete chain by Abd al-Razzaq (7467), Ibn Abi Shaybah (3:105), and al-Bayhaqi (4:228). See also Taghliq al-Ta'liq (3:169).