[pluma]

I'm German. I would strongly advise to talk to a lawyer in this situation. Lawyers are very affordable, especially if you just want to check to make sure the law is on your side.

I'd just look for a lawyer specializing in employment contracts and pay for a <1hr visit to ask them their opinion on the situation. They can also tell you whether it's worth pursuing legally but the most important thing is that you'll know better next time and be able to say "No, you can't do that, you have to pay me for X weeks" if that's what the lawyer says is your right.

EDIT: Having a lawyer is handy if you need to sue someone but the real value comes from knowing with confidence when you're right and when you're not. This defends you against FUD, which can be powerful.