Need help with the game logic editor expression controller

[Jedijds]

I want to use that controller for a money property so that when I buy
stuff my money doesn’t drop below 0. I have it all set up in a test
blend file (I will attach at the end) the problem comes when I press
the button to buy things (the green up arrow) I end up buying double
or at least my money drops as if I clicked it twice. I would like to
know what is wrong with it? Thanks for any help. (heads up the red
down arrow is to sell, there is no stop on it because I was having
trouble making the stop for the green up arrow.)

http://www.mediafire.com/?lwhbtc8r7a4k363