The 46-year-old promotional sales tour organizer, named only as Jörg H., was returning to his car after a shopping trip in the city, Bild newspaper reported on Monday, when he was approached by the killer and shot several times in the chest at close range.
The motives for the murder remain unclear, though police are not ruling out a contract killing, the paper said.
Police found thousands of euros in cash and expensive jewellery on the victim after the shooting.
The killer, whose image was caught by surveillance cameras, reportedly left the scene calmly after the shooting. He is between 19 and 25 years old, 1.80 metres tall (5 feet 9 inches), has a three-day-beard and wore a gray baseball cap.
When police arrived at the scene Saturday the victim was lying in a pool of his own blood. He died later at a local hospital.
The victim was reportedly involved in so-called Kaffeefahrten, or sales trips typically aimed at senior citizens, in which they are bused or brought to a location and given a sales pitch.
One investigator told Bild that the motive could involve a personal or business grievance, and could relate to the Kaffeefahrten. He said the industry can be particularly cut-throat and lucrative. "[Kaffeefahrten] involve a lot of money, and the gloves are off," he said.