Otona no Hatsukoi Hajimemashita

What would you do if you met your first love again? I did it... Office worker Miri was introduced by a friend and by chance met her prince charming from her school days, Tachibana-senpai. Now an adult, he's become even more handsome... Tachibana's passionate approach leads to the two of them having sex that night. The next day, a new boss is assigned to the company she has transferred to. That boss is Tachibana, the man she had sex with the night before.

(Source: Shueisha, translated)