Variable help

heey long time viewer first time poster here, so ill just go right in to it
im trying to use this variable inside a movie clip if that is even possible… trying to make a money system for a game, maybe im just going at it all wrong any one got any tips? or maybe you know of some tutorials on variables i have googled it but have found nothing to helpful (im using macromedia flash pro 8)

the variable is:

cash = “0”;
_root.cash -= 0;

then a few buttons

button1 adds
on (release) {cash +=20;}

button2 subtract
on (release) {cash -=20;}

and button3 is a buy button
on (release) {
if (cash>49) {
_root.cash-=50;
}
}

this code works fine until you put it in a movie clip here is a link to the fla http://theclaw.fileave.com/moneyinmc.fla

oh and btw! how can i make buttons disappear after use? thx for taking the time to help me out and the tips are greatly appreciated!