Midnight Sadistic

After being drowned in debt due to a joint loan with a friend, Takuma finds himself working under Mr. Koujina, the boss of a black market lending company, after a lot of bowing and begging. Over the years, Takuma had learned to adjust to his job and Mr. Koujina's bouts of unreasonable demands. If anything, the latter gives him a sense of satisfaction he can't explain. When a co-worker's blunder leads him to share a house with Mr. Koujina, Takuma finds himself sexually intimate with his boss. For Takuma, a hidden door inside him has been opened.

(Source: Manga Planet)

Note: Includes one extra chapter.