Do not ever try to change your riel back to USD there, very high chance you'll get counterfeit USD in return

if you pay the vendor in big USD note, some might give you small change in smaller USD note, either you insist on local currency change, or you can accept those, just make sure you spend it all before returning

my friend kena before in russian market many years back, paid the vendor USD20 note for a USD2 item, got 18 piece of USD1 in return
only found out almost half of it is counterfeit 1 dollar according to another banker friend back at the hotel
didn't even know some actually bother to counterfeit 1 dollar, all these while thought it's only applicable to bigger notes