I presume this bird was bred at Dortmund. The same unusual cross breeding happened in the Martin Mere WWT collection a few years ago. The hybrids were known as 'gooders' (goosander x eider).
@Eagle How can I contact you, we would linke to use your photo for a publication concerning the identification of a wild hybrid goosander(=common merganser) x eider- cetainly only with your permission...