I can see why it is frustrating, but I wouldnt say he lied or pulled a dirty trick. 50% of the people who will give an offer on the case are going to ask for it at least 30- $40euro cheaper anyway. Like others said....he is a business, and is making room for reasonable offers while trying to make maximum profit. I hate to side with a dealer over a fellow collector, but overall IMO he did you a favor by breaking up the set and selling you the medal you wanted. You had every right to deny the offer that was given.
If it were me selling it.....I wouldnt have even mentioned the case money......I would have just said "I will sell you the medal alone for XX amount of Euro". That way there arent any issues from the sellers end. But it still sux to feel like you you didnt get the correct deal. I fully understand your point of veiw as well. In all reality, I wouldnt worry too much, he isnt going to get what hes asking for the case anyway, his prices in general are through the roof. Are you going to return the medal?