Province is gone

So,recently,I was negociating a peace treaty,which involed province swap.Unfortunately,I missclicked and I sent the wrong province in the exchange menu.Luckily enough,I noticed it in the few minutes after the message was sent,and I canceled the exchange.But,when I came back from work,I noticed that this province is now belonging to the guy I sent the message to.I did canceled the exchange and the guy is now pretending to be deaf.