Even if it sounds so, DFG "Eigene Stelle" does not fund you personally. You wouldn't even be considered a viable PI without being associated to a university with support from an institute. In Germany, if you do not have full support of at least one full professors, any grant application is almost impossible.
So, you don't negotiate with DFG. The money is given to your university/institute and you negotiate with them. In Germany this is the well known "öffentlicher Dienst" with all its standard tables. No exceptions.
Also all additional money goes to the institution. If they manage this properly, you will get your own "Kostenstelle" where you book your expenses in a self organized way. This is the typical way to access grant money. However, if the institute would get in significant financial trouble -- your grant will be affect together with it (not a very likely scenario, but illustrating where the money actually sits).
You can always leave. But you can't take the grant with you. There are other (much more personal) grants like Helhmoltz Young Investigator, DFG Emmy Noether, ERC starting grant, where you can well negotiate and move around to different places. But certainly not with DFG "Eigene Stelle".
There is no public database with proposals. Proposals are always confidential and are not going to made public. Often there are new ideas in them, right... This is somehow the entire idea of it. You have to know someone who trusts you and shows you his proposal.
- How does the TV-L 13 vs TV-L 14 negotiation with the university work?
The actual table is here: https://oeffentlicher-dienst.info/tv-l/allg/
You will have to negotiate the "group/level" 13/14 with your supervisor, most likely this is the institute leader. It depends on the institute structure and your own future plans if this will be successful. The "group/level" negotiation is a strategic one.
But besides the "group/level" there is also the "step" which only depends on time/experience. The "step" you have to negotiate with the human resources department, you will have to prove them "experience in years" so they will put you in a higher "step". If you are young, this is difficult and you may just end up in "step" 1. The "step" negotiation is very technical and super-bureaucratic.
- Could you elaborate on how the "Kostenstelle" works in this context?
That's just bookkeeping. "Kostenstelle" is a dedicated (bank) account for your project. It is very good to have your own account, it gives you a lot of freedom. But also responsibility.
- If I already have a postdoctoral contract with the target university for the DFG project, what should the "employer authorization/availability" for the DFG application say?
You still need official support from the institute granting you office space and working material, computing, lab-space, as well as integration into the institute. Without an institute to support you, you can't get an "Eigene Stelle". It is not enough to point to an existing contract since times may differ, and also the project may change, etc.
- Is it 100% sure that there is no money-back policy if I don't stay for a certain percentage of the scheduled contract duration? For some European funds, I know that if you quit before 80% of the contract duration is completed, you have to give back all the money you received.
This is not 100% sure. But I would be extremely surprised. Maybe someone else knows? But why not just write a mail to DFG. They will tell you. There is a dedicated person responsible for you at DFG (depending on your research topic)...
- If not the fille in proposal, are at least the proposal modules available online (without actually starting an online application)?
I guess you mean this:
- Is it possible to apply for other types of funds (e.g. ERC, DAAD, etc.) while holding an Eigene Stelle?
Absolutely. I think this is within one of the core ideas to apply for larger grants as follow-up. Of course, with any new application you have to make sure it is not an already existing project, but a new one (follow-up)... Also, avoid "double funding". DFG will not "double fund", thus, if there is already money from a different agency, DFG will not give more to the same research.