Your mails are appearing in the mailing list archive (..) but not in my inbox, so I'm copying the content of your mail here:
> How can we set the exchange rates if the currency is different form our country default currency,
In order to be able to set exchange rates, you need to indicate that you want to use multiple currencies. You can do that in the "System > Defaults"-menu. On the resulting page, there's a "Currency" input box. If you want to use more than one currency, e.g. USD (as the regular currency (also known as "functional currency")) and EUR as the foreign currency, you enter into the currency box (without the quotes): "USD:EUR" (the ":" is the important separator).
When you have that, you can choose the currency when creating an order or invoice. When you choose "EUR" and click "Update", an input box will appear for the exchange rate.
> Let's say the purchase order is to be sent to another country.
Does the above help your case?
Robust and Flexible. No vendor lock-in.