Got a mail from my dealer today. His explanation of what has happened, is still not very convincing (he gave me the wrong amp, but he hasn't got the right amp anymore, cos a colleague of his by accident gave my amp to someone else... ah well...).
Our deal is: I can for now keep using the demo amp, and once my new 110 arrives (which wont take long, he promised), he will come to my place to switch them. Which is a nice gesture, cos I live some 90 km from his shop. I pay half the price now, and the rest when all ends well.
Looks acceptable to me (though I'm not sure I want to do more business with him)...
Right, so now I can concentrate on the music again!