I noticed that for accepting paypal payments, the store base currency must be the same in paypal.

The currency that I want to use is not supported by paypal, so i need to choose usd or euro.

If i set the currency in euro, for example an item is 500 euro, but the user wants to see the price is in usd, he will see that is 673 USD

What will happen when he has to pay? Which price will be passed to paypal? 500 euro or the converted 673 usd?
In the first case, the user will have to pay more, and will be unhappy, because paypal currency conversion is not very convenient.