They are rate, but §8 of the TzBfG (Part-Time Work and Fixed-term Employment Contracts Act) anyone who has worked for a company for at least 6 months has the right to reduce their monthly hours. Companies can only refuse in very dire situations (like the company going bust otherwise). Of course the pay will be proportionally reduced as well.

It's not very widely used, because either some employees fear repercussions by their employer, or the just need the money. I've done it in the past and it's awesome.