Reference: Fataawa Ramadhaan – Volume 2, Pages 650-651, Fatwa No.654
Fataawa Noor ‘alad-Darb – Page 74
Question: Whoever fasts a day which he is required to make up [from Ramadhaan], is he then allowed to break the fast? And likewise, if he was fasting a voluntary fast?
Response: It is not permissible for a person who has made the intention to fast a day which he is legally required to make up to [then] break the fast. This is because once he has made the intention and began his fast, he is required to complete it, since if a person undertakes an extended obligation [الفرض الموسع] then he is required to complete it and not allowed to break it.
Indeed he has been favoured with a legislated extension within which he is required to complete the fast, so once he begins the fast [which he is required to make up], then it is prohibited for him to break it.
As for the voluntary fast, then it is permissible for him to break it because he is not required to complete the voluntary fast. However, it is preferable that he completes it, yet he is allowed to break it and there is no harm in doing that, since the Prophet ﷺ would enter his home whilst fasting a voluntary fast and when he found therein food which had been gifted to them (the Prophet’s household) he would eat thereof and [in doing so] break his fast; So this [action of the Prophet ﷺ] indicates that one is not required to complete the voluntary fast.