This occurs during Slackjaw's side missions. You must complete three tasks; fulfill Slackjaw's first mission, obtain the key of the art dealer's apartment, and get the safe combination. To complete Slackjaw's first mission, just do what he asks and finish the optional quests. The key is owned by the poor woman that is being harassed by a few guards in the alley to the opposite side of the doctor's entry door. You can either get the key at the beginning of the mission by saving the poor girl, or returning here after saving Emily from the House of Pleasure and looting the dead girl's corpse. The safe combination is obtained by torturing the art dealer a bit more. The combination is "1-3-8".