Follow the good solution in Requiem for Dreamers to start this. Be sure to talk to Tranquility before going to The Slums. When you reach Shadowlair: Slums, look at your map to find the location of the dealer, named Serenity. You have no option but to start a fight with Serenity and his 3 thugs here so be prepared. If you manage to drop Serenity first, you do not have to fight his gang. Serenity will see things your way and The Dealer will complete.