I'm unable to prove it but am sure I was just ripped off. In the whole scheme of things 30,000 kip is not much to a foreigner given the exchange rate, but I'm worried that this 'scam' now seems to exist in a country previously untouched by dishonesty. In my case I handed over a 50,000 note for a 15,000 beer and the waiter returned with just 5,000, insisting that I just gave him 20,000.
It's all just small beer really, but when you look someone in the eye with a question and they answer with a bold- faced lie it's rather disheartening.
I don't wish to dissuade anyone from visiting this unique place as it really is an experience, just be careful with your money and make sure you announce the value of the any note you're handing over.