Need legal advice - Being exploited. (Duesseldorf)

I am currently under huge ammounts of stress trying to find somewhere to live and rehome my cat. All because my landlord has decided to use the tiniest of damages to take all of my Protected content deposit. The damage is some small scratching to a upvc door, and i sincerely mean small small scratching. He has informed me he intends to replace the entire door, and the window upstairs and would charge me in excess of my deposit.

I need some advice as to my rights, and if anyone is feeling saintly some back up! I have always been very kind and good to people in my life and i am starting to think it is balanced out by people being cruel and exploitative back. It is hard to stick up for your rights if you dont know what they are and find it hard to express them in german.

Sorry to be so melodramatic but it is putting a large cloud over my life at the moment, i live alone on a low income and things like this spell real doom for me, and financial hell for my family to try and help me. They always do, but why should they pay for the greediness of people taking advantage of me?

Any help is sincerely appreciated.