Exchange Rate supports only 2 decimals, so that will limit what you can consider to be Default vs. Foreign. In your case, you can use 4000 for the Exchange Rate, but you cannot use 0.00025 because of this limitation.
This means that Riel must be the Default currency (no need to define this in SambaPOS since it does not use a set currency), and USD must be Foreign, which you will need to define in SambaPOS.
Above shows what you will use as a divisor when converting from Riel to USD. So when someone buys 10000 Riel worth of product, they would pay 10000/4000 = $2.50 USD
Check out the following link for further information on setting up other requirements, namely:
- Account
- Payment Type
- Change Payment Type
http://doc.sambapos.org/doku.php/en/v3/working_with_foreign_currencies