He effectively binned himself off in January by agitating for a move then. It seems pretty clear that he only stayed on the proviso that he could move in the summer. What complicated matters was that he only wanted to go to Athletico and wouldn’t go anywhere else. Athletico got a transfer ban and also realised that, as there was no competition for him, that they could play hardball over the price.
Conte’s famous email to Costa is a weird one - it was common knowledge that Costa wasn’t going to be around after the summer, so I don’t quite know why it is portrayed as such a surprise. I suspect it is more about Costa trying to save face, as a player agitating for a move tends to be unpopular until he actually leaves.
