Umm... not sure did they exchange second buttons or did he returns the button?

wondering the same damn thing

He gave him his own button, because his jacket was missing the same second button as Takao. So in a sense he was implying that he was returning the same feelings in hope of meeting again someday, and they did meet. Midorima returned takao's gesture with his own and this signifies their bond without need for words.

That would be sooooo nice (⌒▽⌒), i just couldn't differentiate between the uniforms!