Even if he does look like moving, Bismarck can easily buy him off by inviting him to supervise a plebiscite in Schleswig (which those nationalistic halfwits in Copenhagen will immediately slap France in the face by rejecting), and maybe adding a half-promise of a free hand in Luxembourg and maybe even the Francophone parts of Belgium.
How can Bismarck promise Belgium to France? It wasn't his to give and France would immediately come into conflicte with Britain the moment it tries to gain any part of Belgium. France (and Napoleon III) knows it, so any promise of any part of Belgium is basicly worthless.