Did you pick grunt work, and did Don talk about Cassie? You should go to the hideout, leave, and then come back. Cassie should be gone. Go to the Don, and she'll be with him, and she'll be mad at you. He'll offer money. Don't take it, say you weren't the one to find her, and therefor shouldn't be paid. He'll say whatever. You now need to rescue Cassie, but need help from other people.