Niki's manga / #police vs yakuza(1)

I think I've read it in different manga as a side story. It's about a yakuza head of some small group that is conected to a bigger group and a police officer in disguise. He's been working in secret in this yakuza group for 3 years. He's the first guy's right hand at this point. But one day he founds out that this yakuza head is fucking another yakuza head to get a promotion. After that the yakuza head reveals to the secret agent that he knows about him being a police officer. So the cop tries to call his boss (at the pollice station) during his yakuza work (he's supposed to watch the yakuza boss - the head of the biggest group they're part of) just to find out that his boss and the yakuza boss are bodies. So he snaps and kills them both. Then he goes back to the yakuza head just to fuck him. Then the yakuza head bacame the yakuza boss and the cop stopped with being a cop and became his right hand for real now (and sex partner I guess).