You have left out a lot of relevant details, so there are a lot of ifs and whens:
A normal employment contract in Germany is "vollzeit, unbefristed" (full time, unlimited). That contract specifies your salary and it cannot be changed without both parties approval. So changing your salary "just because" without your consent is impossible. The only legally available option to change that contract without the other parties consent is firing/quitting. This includes the normal specified notice periods in Germany, there is no "at-will" here, nobody can be fired on the spot (except for obvious illegal activity like attacking your boss with an ax or something).
Obviously, your "consent" can be had for a price. Maybe you really want that job and they threaten to fire you if you don't "consent". Right now with the developer situation in Germany, threaten to fire a developer is equivalent to actually firing him, because he will have a new job lined up way before his notice period and just quit on the company. No sane employer would do that.
Now it can be done more easily if you are not in the default employment bracket noted above. If your contract is limited (for example for a year) they could easily give you a new contract for the year after with a lesser salary. You can accept, negotiate or decline, it's a new contract all over. Same goes for added bonus agreements. They are mostly limited in time or by conditions and if a condition is the persons performance, then sure, they might get less money. But that's not salary. The salary is unchanged from what is stated in the contract, just the bonus, probably agreed upon in a different contract and conditional, is getting less.
And in the end, those people could be contractors, consultants and/or self-employed. That's another can of worms I won't go into. They probably have non-default, limited, conditional contracts anyway and should discuss them with their lawyer.
To make it short, if you have a contract that says you work as X for Y money until they fire you or you quit, then there is no way that they can pay you less then Y (well, short of their own bankruptcy, that's indeed a valid excuse in court).
If they want to change either X or Y, they need your consent or they need to fire you with the legal and/or contractual notice period.