What is the ruling on making a vow to Allah that whoever breaks it must do such and such, such as saying: "If I commit this sin again, I will harm myself in a certain way"? And if one returns to the sin, must he fulfill what he vowed to Allah, or not, given that Islam prohibits self-harm?

If someone makes a covenant with Allah to abandon a sin and then commits it, they must repent for the sin and for breaking the covenant. They also owe an expiation for a broken oath (kaffarat yamin).

Allah Almighty says: "And fulfill [every] covenant. Indeed, the covenant will be questioned about."

The expiation for a broken oath is feeding ten needy persons, or clothing them, or freeing a slave. If one cannot find [the means], then [it is] fasting for three days.

It is not permissible to harm oneself as a punishment.