Q. 400. If a person breaks his fast due to some excuse, and then the excuse is removed during the day, should he fast for the remainder of the day?
A. He is not obliged to fast. This is because it has been made lawful for this man to break his fast on this day based upon an evidence from the Islamic Law. The Islamic Law allows one who is compelled to take medication for example, to take it, but if he takes, he breaks his fast. Therefore the prohibition of the day does not apply to him, because it has been permitted for him to break his fast, but he is required to compensate for it, and for us to require him to fast without any benefit is legally incorrect. So as long as this man does not benefit from fasting, it is not incumbent upon him.
An example of this would be a man who sees someone drowning in the water and he says: If I drank, it would be possible for me to save him, and if I did not drink, it would not be possible for me to save him, so he drinks and then saves him and he eats and drinks for the remainder of the day. This man is not prohibited from eating and drinking on this day, since it has been made lawful for him not to fast in accordance with the Islamic Law. Therefore, he is not required to fast.
For this reason, if there was a sick person, would we say to him: Do not eat unless you are hungry and do not drink unless you are thirsty? Meaning: Do not eat or drink except due to necessity. We would not say this to him, because it is permissible for the sick person to break his fast.
So, every person who breaks his fast in Ramadan in accordance with some legal evidence is not obliged to fast, and vice versa.
Whoever breaks his fast without an excuse, then he must fast, because it is not lawful for him to break his fast and he has transgressed the prohibition of that day without permission from the Islamic Law. So, we must compel him to fast the remainder of the day and to make up for it.
And Allah knows better.
Q. 398. What are the permissible excuses for breaking the fast?
A. The permissible excuses for breaking the fast are: Illness and travel, as mentioned in the Qur’an.
Among other excuses is that a woman is pregnant and she fears for herself or her child. Another excuse is that a woman is breast-feeding and she fears for herself or her breast-fed child if she fasts.
Another excuse is that a person needs to break his fast in order to save the life of someone. For example, he finds a drowning person in the sea, or someone who is surrounded on all sides by fire, and he needs to break his fast in order to save him – in that case, he may break his fast and save him.
Another such case would be if a person needed to break his fast in order to strengthen himself for Jihad in Allah’s Cause, that would also be a permissible cause for him to break his fast, because the Prophet sallallaahu`alaihi wa sallam said to his Companions, may Allah be pleased with them:
“You will meet the enemy tomorrow morning, so breaking the fast stronger for you, so break your fast.” 
So, if there is a permissible excuse for breaking the fast, and a person breaks his fast due to it, it is not necessary for him to fast for the remainder of that day. Therefore, if it transpired that a person had broken his fast in order to save a person’s life, he should continue to eat and drink, even after saving him. This is because he broke his fast for a reason which permits him to break his fast, so it is not necessary for him to fast in that case, because the prohibition of breaking fast on that day has been removed due to the permissible cause for breaking the fast.
For this reason, we support the most authoritative opinion in this matter, which is that if a sick person became well during the day and he was not fasting, it is not necessary for him to fast, and if a traveller arrived during the day at his hometown and he was not fasting, it is not necessary for him to fast, and if a menstruating woman became clean during the day, it is not necessary for her to fast. This is because all of these people broke their fasts for legitimate reasons, and so on that day, there was no obligation upon them to fast, due to the lawful permission for breaking it at that time, so it is not necessary for them to fast.
This is as opposed to the case where it is confirmed that the month of Ramadan has begun during the day in that case, it is necessary to fast. The difference between the two cases is clear, because if the evidence appears (that Ramadan has begun) during the day, it has been confirmed that fasting on that day is obligatory for them, but they are excused for the time before the evidence became clear to them due to their ignorance of it. This is why, if they knew that this day was a day of Ramadan, it is obligatory for them to fast. But as for those other people whom we have described, it is permissible for them to break their fast, even though they are aware (that it is Ramadan) and the difference between them is clear.
 Reported by Muslim in the Book of Fasting, in the Chapter: The Reward of One Who Breaks His Fast on a Journey if He Undertakes a Task (1120).
Source: Fataawa Arkaan al-Islam by Shaykh Muhammad ibn ‘Uthaymeen (may Allaah have mercy on him), vol 2, dar-us-salam publications