The permissible excuses for breaking the fast are: illness and travel, as mentioned in the Qur’aan. Among other excuses is that a women 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 Allaah’s Cause, that would also be a permissible cause for him to break his fast, because the Prophet sallAllaahu ‘alayhi wa sallam said to his Companions, may Allaah be pleased with them:
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“You will meet the enemy tomorrow morning, so breaking the fast will be 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 due to a reason that 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 women 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 permissible for breaking it at that time, so it is not necessary for them to fast.
This is as oppose 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).