Can You Break an Optional Fast?
Question

A man wanted to fast six days of Shawwal, and on one day he intended to fast but he broke his fast with no excuse, and he did not complete his fast. Does he have to make up this day after fasting six days of Shawwal, so then the number of days that he fasted will be seven, or should he just fast six days of Shawwal?
Praise be to Allah.The scholars differed concerning one who starts to observe an optional fast : is he obliged to complete it or not? There are two views: 

That he does not have to complete an optional fast. This is the view of the Shafi`is and Hanbalis, who quoted the following evidence: 


`Aishah the Mother of the Believers (may Allah be pleased with her) said: “The Prophet (peace and blessings of Allah be upon him) entered upon me one day and said: ‘Do you have anything (to eat)?’ We said: ‘No.’ He said: ‘Then I am fasting.’ Then he came to us another day and we said: ‘O Messenger of Allah, we have been given some Hays (a kind of meal),’ and he said, ‘Show it to me, for I started the day fasting.’ Then he ate.” (Narrated by Muslim, 1154)
Abu Juhayfah (may Allah be pleased with him) said: “… Abu Ad-Darda’ came and he made some food for him – i.e., for Salman. He said: ‘Eat, for I am fasting.’ Salman said: ‘I will not eat until you eat.’ So he ate… and Salman said to him: ‘Your Lord has rights over you, your soul has rights over you, and your family has rights over you, so give each of them their rights.’ Then he came to the Prophet (peace and blessings of Allah be upon him) and told him about that, and the Prophet (peace and blessings of Allah be upon him) said: ‘Salman is right.’” (Narrated by Al-Bukhari, 1968)
Abu Sa`id Al-Khudri (may Allah be pleased with him) said: “I made some food for the Prophet (peace and blessings of Allah be upon him), and when it was served a man said: ‘I am fasting.’ The Messenger of Allah (peace and blessings of Allah be upon him) said: ‘Your brother has invited you (to eat) and has gone to some effort for your sake, so break your fast and fast another day in its stead if you wish.’” (Narrated by Ad-Daraqutni, no. 24; classed as sound by Ibn Hajar in Al-Fat-h, 4/210).


The second view is that optional fasts should be completed, and if a person breaks such a fast he has to make it up. This is the view of the Hanafis who quoted the following evidence that such fasts have to be made up: 


`Aishah (may Allah be pleased with her) said: “Hafsah and I were given a gift of food and we were fasting, so we broke our fast. Then the Messenger of Allah (peace and blessings of Allah be upon him) entered and we said to him: ‘O Messenger of Allah, we were given this gift and we wanted to eat so we broke our fast.’ The Messenger of Allah (peace and blessings of Allah be upon him) said: ‘It doesn’t matter; fast another day in its stead.’” (Narrated by Abu Dawud, 2457; At-Tirmidhi, 735. Its Isnad (chain of narration) includes Zamil, of whom it says in At-Taqreeb: he is Majhul (unknown). It was classed as inauthentic by An-Nawawi in Al-Majmu`, 6/396 and by Ibn Al-Qayyim in Zad Al-Ma`ad, 2/84. It was also classed as inauthentic by Al-Albani.
With regard to the Hadith of `Aishah quoted above from Muslim, some of them add the words: “… ‘I started the day fasting,’ and he ate and said: ‘I will fast another day instead.’”

It should be noted that An-Nasa’i (may Allah have mercy on him) classed this addition as inauthentic and said: This is wrong. It was also classed as inauthentic by Ad-Daraqutni and Al-Bayhaqi. The first view is the one that is more likely to be correct, because its evidence is stronger. And it is supported by the report narrated from Umm Hani’ (may Allah be pleased with her) who said: “O Messenger of Allah, I broke my fast and I was fasting.” He said to her: “Were you making up for a day you had missed?” She said: “No.” He said: “Then it does not matter if it was a voluntary fast.” (Narrated by Abu Dawud, 2456; classed as authentic by Al-Albani) 
Shaykh Ibn `Uthaymin (may Allah have mercy on him) said: 
“If a person is observing an optional fast and something happens and he has to break his fast, then he should break his fast. This is what was narrated from the Prophet (peace and blessings of Allah be upon him), that he came to the Mother of the Believers `Aishah (may Allah be pleased with her) and said: “Do you have anything to eat?’ She said: “We have been given some Hays (a kind of meal). He said: “Show it to me, for I started the day fasting.” Then he (peace and blessings of Allah be upon him) ate some of it. This has to do with optional fasts, not those that are obligatory.” (Majmu` Al-Fatawa, 20) 
Based on this, you do not have to make up the day when you broke your fast, because the one who is observing a voluntary fast is in charge of himself; rather he should complete six days of Shawwal . 
And Allah knows best.