This page of the Disco Elysium guide contains a detailed walkthrough of the side quest: Buy FALN Pants from Cuno.
In the Buy FALN pants from Cuno quest in Disco Elysium, you have to purchase FALN tracksuit pants from Cuno.
Before you can buy FALN sweatpants from Cuno, you must find the victim's belongings first that have been thrown into a dumpster (Open the trash container quest) and then talk to Cuno about these items (Who put the clothes in the trash? quest).
In the course of the conversation, take an interest in Cuno's pants by selecting the dialog option: Alright, entertain me - what's so great about these pants? The boy will tell you about the pants and then you can buy them from him for 15 Real.