The Riel per US$ exchange rate on the streets of Siem Reap or Phnom Penh has been stable at around 4,100, but most shops will round this down to 4,000 for ease. When receiving change from a dollar, the cents will be returned to you in Riel because the Khmer people do not use American coins. For example, if you give a 10 dollar note for a US$2.50 item, you will usually get US$7 and 2,000 Riel change.
In Cambodia’s countryside and local markets, Riel is almost popular than US dollar, but you are also able to use dollars in small denominations. Do not expect anyone to change a US$20 bill for a $0.50 purchase, so remember to bring a lot of 1 dollar and 5 dollar notes. They are very useful for tipping and buy some small and cheap items. Your US dollars will be rejected if they are ripped, torn, or otherwise overly abused. Old-style US notes are not also welcome, so make sure that the cash you bring is fairly new ones.
There are many ATM machines in Cambodia which dispense US dollars. You are no need to bring a lot of cash with you, just bring a foreign ATM card. It will be enough.