I thought this was a joke post but the OP is actually serious, get a life. If you are that cheap you could have gotten your dollars upfront outside of the country too, and once you have dollars, you can just pay in dollars anywhere too, no need to change it. Whining over 120 riels, 0,029 USD cents lol.
Even you would lose 20 dollars on your exchanged money, you get cheap beers that earn it back already after 10.