[Xylakant]

You're not free to set your own price, there are guidelines set forth in the law (ArbnErfG, § 11 Vergütungsrichtlinien) - which also means that the employer cannot undercut those guidelines and force you to hand your invention over for free. If there's a disagreement about the price, there's a mediation set forth in the law, so the first thing would not be a court case.

In general, german courts are quite employee friendly, so seriously, your employer would probably be at least as afraid to go to court as you'd be. Maybe not if it's a litigation-happy megacorp, but the bulk of german companies is medium-sized (Mittelstand) and they tend to shy away from court cases.