Is the method of avoiding sins by writing a document in which you commit to paying a sum of money for each sin considered legitimate and sound, taking into account that the avoidance of sins might be due to fear of paying money, not fear of God?

It is permissible for a person to penalize himself financially for committing sins, citing the action of Ka'b ibn Malik, may Allah be pleased with him. One should not pay attention to the feeling that abandoning sins was out of fear of giving charity, because abandoning sins is good in all circumstances. The true motive behind committing to the amount is the fear of Allah, so one should not pay attention to the whispers of Satan.