This is a common trick in a lot of places
They did that to me once in Argentina during a business trip and I learned my lesson.
Happened again to me in France, Mexico and Italy, with taxi drivers and at a restaurant... except I took a picture of me handing the bills without the other person noticing.. shock & embarrassment looks everywhere when trick was attempted.
The lesson is: there's scammers everywhere in the world. Be vigilant, be careful and always, always take pictures.