A code workaround for 'provided item'?

I have an item that has 10 ''uses" and when a button in a GUI is clicked the item's durability will go down. I have everything set up in a procedure, but the ''on button press" event handler does not have support for itemstacks. The GUI is bound to a block, the item goes in a slot in the GUI. The "itemstack" dependency comes not from actually moddifying the damage, but from detecting the current damage and breaking the item when it reaches -1 (because that is how Minecraft handles it, for some reason, and I want to be consistent). I suspect the problem is that the only procedure block able to detect damage is "get damage of the provided item," so is there a way to define "provided item" without mucking around in the code? If not, could someone tell me how to do the proper code manipulations? (I know JavaScript, but I am nowhere near familiar enough with Java or Minecraft's objects to know what I'm doing)