hello thank you so much for reply.
I use the 3.70 version of nopCommerce. The problem is not about converting currency. i use ECB exhange rate provider and everything is ok.
My primary currency is EUR. If a customer sees in USD when he goes to pay through paypal express the currency is "USD" while the value of the amount is in EUR
I have to try to use it for now duplicate the entire site and DB .. but is not comfortable .. :-)