If I get you right based on your last response its about getting the right exchange rate to use or is it that you want to do cross exchange of Fiat currencies?
Thank you for your reply! Yes, I have to use fiat (EUR) as my base currency for now. The two-step transaction makes a lot of sense. How would you determine the exchange rate? I guess the BTC-Euro rate at the time of the exchange to alt? Or at least a best estimate if I miss the order going through.