Yeah, I had expected something along those lines. That's why I didn't hand him a 50 in the first place, but two 20s. A few days earlier in Istanbul another cab driver tried to pull that trick on me after I paid him with a 50 Lira bill. He quickly pulled a 5 lira bill and told me that the fare was 35, demanding an additional 30 lira. I told him to check his other hand, where we would find my original 50 lira bill
I got my 15 liras in change and thought to myself that these guys are somewhat idiots for choosing to forgo a tip. I guess they "win" more often than not and that's why they continue doing it.