Step by step
The riel total is rounded
The dollar total is converted at today's rate and rounded to your riel step (Settings › Sales rules), because nobody pays ៛31. $16.09 at ៛4,100 is ៛65,969, shown as ៛66,000.
Either total pays in full
Exactly $16.09 is enough, and so is ៛66,000. Mixed is fine too: $16 and ៛400.
Change is in riel
Change is rounded down to the riel step, so it is never more than what is owed back. $20 for $16.09: ៛16,000 change.