Is it cheaper to buy a BMW in Germany with euros and have it shipped to America or buy it in America with dollars?

It will probably cost you more shipping the car over from Germany, because the car was built to European standards not American. Why don't you look into the European Delivery Program? I know Mercedes-Benz offers you a discount on the vehicle, then you go pick it up from Germany, drive it around Europe for as long as you want then take it back to any of the designated ports and they'll ship it to America for you.