This one requires lot of experience.
1. go find and kill hargvar the hairy without killing any of his gaurds.
2. After hargvar is dead kill one of his gaurds and travel to the castle.
3. There will be a man you havent noticed before in here talk to him and say"who are you?" after that he will tell you he's a world wide theif and will give you money if you dont report the gaurds. Say "No i dont think so". He will attack. Run out and find a alpha snarl. You must let the alpha snarl kill the man. If the alpha kills him you get 20,000$. Repeat this as many times as you like.