Otonari-san no Yoru no Kao

"How much do you know about your neighbor?"

Kirue Kiryu... the breath of fresh air who lives next door.

Yukio (protagonist) and Kirue cross paths daily, but recently they've become closer friends.
But Yukio then saw a side of Kirue he shouldn't have...

Certain circumstances led Yukio to a paid sex house, where he discovered Kirue was working at night.

Feeling suddenly disgraceful and impure, he nevertheless paid her to be a companion.

She was a true professional. And thus their relationship entered a new (client + service) phase.

[From DLsite English]