Detecting when an item is placed in a GUI slot

I'm currently working on a trade system similar to villagers using an entity and I want it work like so:

You put an item in the input slot. It detects that it's there and gives you an item in output slot. If you take the output you lose the input.

I'm having trouble correctly detecting when an item is inputed into a slot in a GUI. I used the "When slot contents change" procedure call but that detects the slot before the change and not after, so when I say if item is in slot, it ends up false and when I remove the item, it turns true and functions as normal. I tried saying if not item is in slot, but that made the procedure go off as soon as the inventory was opened.

Using a button won't work for what I want to do because I need the item to be visible to you when you input your currency item.