What is the ruling on paying an expiatory oath (kaffarat yamin) for a covenant made with Allah to abandon masturbation in exchange for a charity of 100 riyals for each instance, knowing that I was unable to commit to the charity after a period? And does the covenant remain valid?

This type of vow is called a "vow of obstinacy" (nadhr al-lajaj), and its maker intends thereby to prevent himself from something or to urge himself to do it. The preponderant opinion is that the one who made the vow has a choice between offering expiation for an oath (kaffarat yamin) or fulfilling his vow. As for the repetition of the vow with the repetition of the action, its ruling has been previously explained.