I would like to set folder permission to allow a user to save files in a folder but not to rename or delete .. is that even possible ? As far as I understand the linux write permission it allows also delete / rename ?!?

I don't think that is possible with standard permissions. You can set the "sticky bit" on a directory which allows only the user that created/owns the file in the directory, the owner of the directory and root, to delete/rename it. You would do that by giving all users rwx permissions on the directory then set the sticky bit with:

# chmod +t <directory>

The /tmp directory typically has this type of permission, i.e. everyone can write to /tmp but only the person that created the file(and root) can delete it.
That's the closest that I think you can come with standard permissions. You may be able to get more fine tuned permissions/control with ACLs(short for "access control lists") but I'm not that familiar with those.