Excuse me, but I think you've overcharged me.
How much change did I give you, sir?
I'm very sorry for the mistake. Here is the right change.
I'll check our accounts. Just a moment, please.
Could you check again, please, sir?
I'm afraid I was short-changed.
I'm terribly sorry.
Waitress, l believe you haven't given me enough change. I gave you a twenty.