What is the ruling on a fasting person who sleeps before Maghrib and does not break their fast, then wakes up after Isha?

There is no harm in sleeping before Maghrib. The fast of a sleeping person who did not break his fast at sunset is valid, and nothing is required of him, because breaking the fast at sunset is not a condition for the validity of fasting. Rather, the Sunnah (definition) is to hasten the breaking of the fast. It is disliked to deliberately delay breaking the fast until after sunset, but this disliking does not apply to a sleeping person because the pen is lifted from him. If he wakes up after Isha, let him break his fast, and he must pray Maghrib then Isha.