Being in charge of finances

So my fiance is horrible with money. He spends beyond his means to the point he/we struggle to pay Bill's each month. He had been without a car for a while and was using mine to commute to and from work. With income tax we agreed to take 2500 to get him a car. After talking and deciding on what car to get, we decided we should invest in a newer vehicle that would last the family a while. (We have a couple small children.)

Because of a car accident he had gotten into and was at fault for and having horrible credit because of his wreck less spending, in order to get a decent car that would be worthwhile and affordable I would have to put the car in my name.

After a lot of consideration and talking we decided that I would get the car in my name so he could have low payments and affordable insurance. But a stipulation of doing this I told him he needed to allow me to be in charge of the finances.

Each week he takes 80 out of his paycheck for spending money through the week, for gas, coffee and whatever else he may want throughout the week. And from there I pay the bills and start a savings account. This way we aren't struggling anymore.

Has anyone done something similar to this? I'm hoping it doesnt bite me in the ass in some way. I made sure I could afford the car on my own if God forbid we were to split for whatever reason. But I'm worried about possible anger and disagreements if he wants more money out of the account or something. Is 80 a week a reasonable amount for miscellaneous spending money? How would you go about a situation like this?