From what I remember a lot of clubs did come early, and €100m is a massive return on the little invest Dortmund made.
And Dembélé wasn't interested, that's an important part. Also 100m isn't massive at all for Dortmund, you just have to look at prize money and TV righs, if Dortmund don't hit their targets, Démbélé can easily become a 40m or less sell. Just as an example Monaco made 40m with CL in 2015, you don't qualify and that's what you lose.