His Secret Office Romance Partner is Me.

Salesman Ayakawa and designer Yamagishi are notorious for their constant bickering at work! One day, Ayakawa hears from a junior colleague that Yamagishi is in an office romance—and it instantly puts him in a foul mood. After all, he is the one dating Yamagishi! Behind closed doors, the two are actually a sweet and affectionate couple. What happens when their secret office romance comes into play?

(Source: Core Magazine, translated)

Note: Includes one extra chapter.