Money changers display two rates. The larger figure shows how many riel must be paid to buy a dollar note. The smaller figure shows how many riel you will get if you are selling dollars. The trick to getting a better rate is to look for the two numbers that are the closest together. For example, if the exchange rate is 4010 riel to a dollar, a money changer simply out to make money might charge 4050 riel to sell you a dollar and 3990 riel buy a dollar from you. That is quite a spread and she makes a lot on each transaction. The more friendly dealers might charge you 4015 and 4005 riel respectively. That’s what you look for.
[Note: These photos were taken on different days so the base rates can vary widely. What you want to pay attention to is the difference between the two rates.]