The con artist is, as his name not so subtly suggests, not someone to be trusted. Geralt meets this shifty character just outside the Temple Quarter entrance to the sewers. The witcher can choose to accept a proposal from the scoundrel and get some loot at the same time. He appears at 6 am. After accepting the proposal he is found roughly 2 hours before Dawn.
In addition, if the player elects to play judge, jury, and executioner on the con artist for being conned after the quest, it is possible to kill him. Simply meet the npc a little earlier than 6am, hand in the heirloom and as he is walking away, attack him. This action will not incur the wrath from a bounty hunter.