i have the problem, that i need to exclude the path "/backup" from the user and group-quota. my problem is, that this path is at the same filesystem than the userdate itself, so i can not exclude a file-system, i need to exclude a path. can anybody help me (or tell me, that this is impossible)
I don't think this is possible - you'd need a new partition. If you don't have enough space to create a new partition, you can create an image file and then loop-mount it.