Bank error in your favour - collect $800

Today we exchanged some travellers cheques at a bank here. I had two $100 Canadian cheques, and I mentioned to the guy (who could speak a bit of English) that they were Canadian dollars. Well, somehow he got confused and doled me out the equivalent of $1000 US in Korean won. I noticed that that was way too much money, but at that point he had started processing Brian's cheques. We had pointed out that Brian's cheques were in American dollars, and then he realized he'd made the mistake of calculating mine in American dollars. He also caught his mistake on the amount.
I would've told him anyway (though I was mentally calculating that he didn't have very much information about me... just my passport information ..... perhaps had I been a bit quicker getting out of there .... ).