What is the ruling on one who made a covenant with Allah to marry an orphan girl, then broke his covenant and married someone else; does he owe an expiation or repentance, and is this covenant considered a vow?

Allah Almighty commanded and urged the fulfillment of covenants. The Prophet, may Allah’s prayers and peace be upon him, emphasized it. Therefore, a Muslim must observe and preserve his covenant as long as it does not involve disobedience, especially if the covenant is with Allah. If the covenant is for a legitimate purpose, such as sponsoring an orphan girl or caring for her, it must be fulfilled. If one does not fulfill it, he must offer expiation for breaking an oath.