Asahi Ranko's Manga List

A girl appeared before him one day, wearing a collar and leash. He considered turning her away, but instead gave her the name Kuro and kept her as his pet.

At first, he assumes she has been put up to it as part of some role-play with another man. However, as they spend more time together, the situation escalates, and he begins to see her as his own personal submissive dog.
After training her clitoris with his fingers and vibrators, he takes her out for a walk, leading her by a leash attached to her enlarged clit. When they encounter two strangers, an orgasm endurance game begins.
Kuro loses despite knowing the stakes, and they record her vulgar orgasms in the middle of the street...