Cruz is my picky eater and he does exactly the same thing! If I sneak in a little extra, he will leave that exact amount in his bowl! How DO they know?
Canned tripe mixed in works pretty well for him. He likes Solid Gold better than Tripett. Last week I bought a can of Kiwi Peak tripe (lamb and venison) and he's completely mad for it right now. Won't last long though.