You counter that with:

> Well, I have no information about that. What counts for me is the opportunity costs, i.e. what I could make on other engagements. I love my work and I love this company, but unfortunately I can´t afford to lose money just to work here. So can we please concentrate about the value I have to offer?

BTW: German law supersedes any contract you might have so you can talk to your colleagues about salary. Reference in German: [Was das Arbeitsrecht zur Schweigepflicht beim Gehalt sagt][1]


  [1]: https://www.haufe.de/personal/arbeitsrecht/schweigepflicht-ueber-das-gehalt-reden-verboten_76_348410.html