Set an item in the item's inventory

Hi, I got a problem with my procedure. I have an item (let's say backpack) with inventory. And I want to put it in block's GUI and by pressing a button set an item (obsidian in this case) in the backpack's inventory. The problem is that the procedure doesn't set obsidian into backpack's inventory. I tried it backwards, so the procedure copy an item from its inventory to blocks or gets its name but I tried everything and none of my tries sets an item in the backpack's inventory. I would be grateful for any help.