Well, I wouldn't want to tick him off, but tell him in nice words that you have been looking for this set and that you would like to keep it. And tell him you will send something to him in return. (A couple green coaches or something that he would want) and if you are lucky, you will get to keep the set. The Donald and Douglas snowplow set is extremely rare and I wouldnt suggest ripping him off, but at least try to keep the set.
Proud owner of Tomy/Plarail trains.