Seems like a cool idea, but I'm not sure if it would motivate me. If I don't spend on something now usually I'll just end up spending that money later. It would be great if the money moved to another account when I say that I'm "keeping" the money
It makes more sense, imho, something that collects till a goal and then makes a single bigger transaction. The goal can also be useful for motivation (3 more coffees and you reach your 100$).