Make your way across the bridge and to the east, then activate your Witcher Senses to find a puddle on the ground. Speak with him and agree to recover the papers to begin this quest.
Head to the Tretegor Gate fast travel point to find Rostan Muggs wandering about nearby. Witcher 3: The Dwarven Document Dilemma Quest Walkthrough Witcher 3 Novigrad Side Quests Walkthrough