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Hi!
You have to click on the old cover in Zanthias cell. After the third click Zanthia will create a rope out of it and Marko gives you a hook which together make a fishhook. Use the fishhook on the window and Zanthia will get the fish with the key. That´s it. The sheriff won´t come back. I love this part.
Btw, you can also use the fishhook when the key is still on the wall, if you happen to have no magnet!