I believe there simply is a confusion of what exactly you asked for. In Germany, I believe it's less common to write letters of recommendation in the US style. Instead, what one typically does is list 2-3 contact persons in the application, and if the application is seriously considered some or all of these references may be directly contacted by the future employer (by phone, email, etc.).
Usually, this is seen as more of a sanity check than a deep evaluation - i.e., the referee will mostly be asked if the stuff that you list in your application is factually correct. Some people will also ask for a subjective evaluation, but most employers are rather unwilling to go there since you are by law not allowed to give a bad reference, so most people will stick to the facts.
That is to say, if what you want your professor to do is to write a detailed letter of recommendation you maybe should check if that's also ok (since that's a lot more work than what would be expected if one is listed as a reference in the German style).