according to http://coinmill.com/KHR_calculator.html 10,000 riel is equal to $2.40. take as many small dollar bills as you can ($1 - $10) as small businesses won't have change for anything bigger. a lot of the time they will want to give your change in riel even if you paid in dollars. you can very quickly end up with a pocket full of notes. i didn't try it, but i would imagine 50s and 100s would have to be taken to a bank.