What should one who curses another while voluntarily fasting do; should he break his fast or complete it, and should he make up that day's fast?

A Muslim must complete his fast despite committing sins, due to the generality of Allah's saying: "And do not invalidate your deeds." His fast is valid, though its reward may be diminished. He must repent and seek forgiveness. If the fasting person repents with sincere repentance, it is hoped that he will receive the full reward, for repentance removes what occurred before it.