What should a person do who vowed to send a monthly sum to the family of a deceased worker he employed, but then became unable to fulfill his vow due to his circumstances? Is he obliged to send the monthly sum, or should he expiate for his vow? And what is the expiation for a vow, if any?

An oath of obedience must be fulfilled, for the Prophet, peace and blessings be upon him, said: "Whoever vows to obey Allah, let him obey Him." If the one who made the vow is unable to fulfill it, the sin is lifted from him, for Allah Almighty says: "Allah does not burden a soul beyond its capacity." However, he is still obligated to offer an expiation for breaking an oath (kaffarat yamin). So, whoever vowed to donate a monthly sum and was unable to do so in some months, he must offer an expiation for breaking an oath for those months, and he must continue to fulfill his vow whenever he is able. But if his failure to fulfill it was due to negligence, then he must make up for what he missed.