What the dealer failed to notice or tell you, is that Merciers are French bikes, and yours therefore had a French 25mm fork. That means the thread, and stem diameter are different than the 24tpi fork he sold you. Now you'll need to replace the stem with a 22.2mm one. Unfortunately it also means that you'll likely have to change the bars to match the stem.

It's a case of the shin bone is connected to the knee bone.....

You could have avoided the hassle by sourcing a French thread fork at the time, but they're hard to find new these days. In any case you crossed that bridge long ago, and must now continue to move forward.