i cant make a procedure that damages an item

hi,

i am creating a magic mod where there is a wand where you can put spells, and when put the spell in the slot and press the button ''cast'' it should give you back the item, but damaged. i tried with the ''deal x damage to provided itemstack'' block replaicing the block ''provided itemstack'' with a block that contains the spell item but when i try it in-game it wont work.

can someone please help me?

thanks