The desperate merchant can be found wandering up and down the Vizima dike in Chapter III. It seems that he is in need of the services of a witcher to retrieve a family ring lost in the swamp. Apparently his relative was attacked by monsters near the Melitele's shrine and when he tried to run away he only fled farther into swamp and the path of danger. Needless to say, he perished and with him, somewhere in the swamp is a valuable ring.