He should not break his fast; because it is possible that the sun has not set, and breaking the fast would invalidate the fast. If he breaks his fast while in doubt about the sunset and it later becomes clear that it has not set, he is required to make up the fast. This is different from the fasting person who eats with doubt about the dawn; if it later becomes clear to him, he is not required to make up the fast. The reason for the difference between this and eating before dawn is that night is the original state there, so day cannot be established by doubt, and thus the certainty is not invalidated by the doubtful. Here, day is the original state, so night cannot be established by doubt, and breaking the fast occurs during what is considered day, thus he must make it up. If he is mostly certain that it has not set, there is no doubt about his obligation to make it up; because the ruling of the original state, which is the continuation of the day, adds to his predominant assumption, and his breaking the fast occurs during the day, thus he is required to make it up. And he is not required to pay a penalty according to the correct opinion; because the possibility of sunset remains, so the doubt is established, and this penalty is not required in the presence of doubt. See: Badai' al-Sanai' 2: 105-106.