In good practice you would create a get/set interface and in there say something like
if(!money<=0) money += mny;
where mny would be the parameter given by the set function. Or you could just make a global var and function that is called whenever you modify money. It's the same deal, you just don't want to have to change all your money-modifying-scripts whenever you make a small change or have to type it out again and again.
