What is the ruling on vowing an amount of money exceeding the legal expiation for one who broke his oath, and is it obligatory to fulfill it?

A Muslim must repent from all sins and regret them. If they return to sin, they must perform sincere repentance (Tawbah (definition) Nasuh). If you swore not to commit a sin and then committed it, you have fallen into two prohibitions: committing the sin and breaking the oath. The first requires repentance, and the second requires expiation (kaffarah (definition)). As for your vow to double the expiation if you break your oath, it is an affirmation of the oath, and its ruling is the same as that of an oath. You have the choice between fulfilling the vow or offering a single expiation for breaking an oath. Therefore, you have the choice between doubling the expiation according to your vow, or offering a single expiation for breaking an oath (feeding or clothing ten needy persons), in addition to another expiation for the first oath you broke. If a person swears not to break their oath and then breaks it, the expiation multiplies for them.