Anywhere in the country you can pay for something in dollars and get your change in a mixture of dollars and riel. This is a bit confusing at first but you’ll soon get the hang of it.
Using dollars to pay in shops and restaurants is convenient but the common street rate of 4,000 riels to the dollar often slightly undervalues the American currency. Take your large notes to a jeweller to get more bang for your buck. Before you head off the beaten track, get a stash of small change because rural vendors may struggle to split big bills.
Any old-style U.S. notes may not be accepted and shops are also wary of large denomination bills with rips in them. When it comes to other currencies, it’s easy to pay in Thai baht near the border crossings but less so elsewhere. Euros are not commonly used. You can change both of these and many other currencies in banks and exchange shops.
Credit cards are rarely accepted in Cambodia except in upmarket hotels, shops and restaurants.