Back when Nick Wilde was the protagonist of the film, Koslov owned a secret bar/restaurant in Tundratown that was a front for his criminal activities.
Nick was trying to get funds for building an amusement park called Wild Times, but no banks would loan him the money because he was a fox. Finally, after being rejected by every bank in Zootopia, Nick resorted to his final option - Koslov, a notorious polar bear criminal.
Accompanied by Clawhauser and Honey Badger, Nick visited Koslov's Palace and begged Koslov for the money. After Nick explained that he was building Wild Times so that predators could remove their tame collars and be themselves, Koslov agreed to loan Nick what he needed.