The settings contains comma ',' asdigit separator. When I entered the recent FX rate, like this: 309,38 it saved to the DB in the right form with the right value.
When I got back the Sales Offer screen, it displayed the exchange rate as 30.938
When I add new item to the offer, it seems it calculated with this invalid number.
When I saved the offer and I created a list about open sales offers, the list contains the value of the offer in HUF correctly, so, the internal calculation seems ok, except displaying already entered values.
I had no time to dig deeper and fix the code.