I made a vow to Allah Ta‟ala that each time I commit a certain sin, I will give X amount as Sadqah.

Answer

The vow is like a Yameen (Oath). Each time you commit the sin, you have to either pay the stipulated amount or pay the Kaffaarah Yameen which is to feed ten masaakeen (poor Muslims). But there is no way of cancelling this vow/oath. Every time you violate it, the ruling will apply.