1 bolean or variable on the item
tripple condition :
Trigger once
is bolean X inverted/variable=0
cash =>500
action --> subtract 500 from cash
------------> set bolean true /set variable to 1
this should solve it.
i placed bolean and variable with a / so you can choose the way you want it... it doesnt require both just one of the solutions.
here is a capx example using mouse and instance bolean