Professor uses academic employees to write proposals
If I understand correctly, you (as a postdoc) developed a research proposal, your professor submitted it under their name, and the project was funded. Then you were sidelined --- you will not be a PI on this project and you will not even take any role in it. This is a terrible experience and I am sorry to hear it happened to you.
The professor's behaviour is wrong. Even for German academia, notoriously famous for being professor-centred and often exploiting young researchers unfairly, this is probably beyond what can be considered normal.
Now, what can probably happen if you expose the story? Likely, not much. There are two issues here:

It is impossible to verify reliably who was the author of the proposal. Even if you show the time stamped drafts, it only proves that you typeset the material and prepared it for submission. Your former professor may say that they explained you the main idea and asked you to type it. It's very hard to prove authorship.
Even when evidence is quite convincing, most academics and academic administrators won't pursue an established professor for a single case of academic misconduct, if it does not directly hurt the University. Ideas are stolen very often, but such stories rarely made headlines, so most administrators won't bother dealing with them. Of course, there are exceptions, when even a single problem can unexpectedly grow big and eventually cost academic their job (there were several such stories recently related to hot topics in political/social discourse). But as far as I see quite a few professors made their carriers by borrowing ideas of others and making their students and postdocs to do a lot of their own work.

So, what can you take from this story? First, you know that you can develop a winning research proposal. This is a sign of a great researcher! Not many people can do it in their postdoc. This is something you should be proud of, and this skill and dedication is something no-one can steal from you. If you decide to leave this story behind and focus on what's ahead, I hope you will be able to repeat this success and now get the full credit for your hard work and creativity.
Good luck!