The black haired guy left an envelope with money and a note saying "the summe according promised" implying that Souchiro had Chiaki over on an agreement with a external party (probably Chiaki's grandpa). With this, Chiaki would think that Souchiro was treating him well for the money, not because Souchiro liked Chiaki, hence, Chiaki would be hurt and wouldn't trust Souchiro anymore.
He made it look like the Seme got paid for taking care of the Uke and that the Seme bought Flight tickets to Tokio for the Uke.
So basically:
"The whole time I was just nice to you because I was getting paid by your grandfather. Me getting tricked that a wife candidate was coming and that I had to take care of her was just a lie. I got never tricked. I knew that you would come. I did it just for the money. And now that I got paid, I don't need you anymore. So I bought the ticket.
this is so mean!! why do this at all!
I couldnt understand what he black haired guy did can someone explain it to me