Sachiiro no One Room Gaiden - Seikai no Meitantei

This is the story of Detective Matsubase three years before he fatefully meets xxx.

Hijiri Matsubase finally quits the police force and fulfills his dream of running his own detective agency. However, life isn't smooth sailing as it seems and the poor guy is in massive debt after getting scammed. Luckily, or unluckily for Matsubase… Aketoyo, the son of a billionaire reaches out to him for a deal. "Entertain me, I'll pay you $100,000 per case." A deal with the devil can be oh so enticing.
(source: AOC Translations)