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.
Yes, the BTC-Euro rate (from any exchange) would have to be used. If you miss the time of the order going through, an estimate would be sufficient.
If you know the time of the transaction, there are exchanges which give you detailed minute by minute graphs of historical bitcoin prices. Of course, if you aren't finicky, you don't need those.