Reference: al-Muntaqaa min Fataawa Shaykh Saalih al-Fowzaan – Volume 3, Page 135
Question: Is it permissible for one to break the fast which was [legally] required to be made up? Likewise with the naafilah fast?
Response: It is not permissible for a person who intended to fast a day which was legally required for him to make up (i.e. from Ramadhaan) to then break it (the fast). This is because he had made the intention and then started to fast. [In this case], it is then obligatory for him to complete the fast for that day. This is because if a person sets out to do that which is obligatory upon him, then it is obligatory that he completes it and he is not allowed to break it. The time (period) within which to carry out this act is unrestricted so when he begins the act he is therefore not allowed to break it.
However,if he fasts a naafilah fast, he is allowed to break it. This is because it is not required to complete the naafilah fast. Rather, it is better for him to complete it, however, he is allowed to break it and there is no harm in him doing so. This is due to the Prophet (sal-Allaahu ‘alayhi wa sallam) one day entering his house whilst performing a naafilah fast and when he found therein some food which had been given to him, he (sal-Allaahu ‘alayhi wa sallam) ate from it. [Thereby], he broke his fast. This indicates that it is not imperative to complete a naafilah fast.