Is one who pledged to sponsor two orphans then became unable to pay their monthly installment sinful?

If your commitment is a covenant with Allah, then it is a vow and an oath that must be fulfilled, especially if it pertains to an act of obedience. If you are temporarily unable to fulfill it, you are obligated to fulfill it later when you are able. However, if it is merely a personal commitment from you, it is not considered a binding vow. Sponsoring an orphan is recommended, not obligatory, and a servant will receive the reward for their good intention even if they are unable to perform the deed.