He makes many of the girls who came from Svelera become working girls in his brothel. Nina and Izura, the girls that Yuuri meets in Hildyard and hit on Conrad are, in the manga, about 12 years old.
Hyscliff, on behalf of his wife's family who have business in Hildyard, wants him to stop. Biron decides that he wants to bet on it. If he wins he'll get Yuuri and sell him to rich people, if he loses he'll stop making the girls work as prostitutes.