What is the punishment for one who vowed to perform an act of obedience, then became negligent and failed to do so?

There are two types of vows (nadhr): conditional and absolute. Both must be fulfilled if they involve an act of obedience, based on the Prophet's (peace be upon him) saying: "Whoever vows to obey Allah, let him obey Him." The obligation to fulfill a conditional vow is even stronger. The Messenger of Allah (peace be upon him) condemned those who make vows but do not fulfill them, describing them as treacherous and untrustworthy. Failure to fulfill a vow is a characteristic of hypocrites who made a covenant with Allah and then broke their promise, which results in hypocrisy in their hearts until the Day they meet Him. It is feared that Allah may punish whoever makes a vow and does not fulfill it by casting hypocrisy into their heart.