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











Omg, i love the manga, but what makes me love the whole thing more are the last 2 pages ヾ(☆▽☆) they are so funny and so spot... just perfect! (๑•ㅂ•)و✧