What does Sporting do in the contract then?
There is nothing to worry about. The clause apparently was in Joao Mario's transfer deal that if we sell Joao Mario to another Portuguese club, we need to pay the penalty. It's still unclear what the clause was in detail (though I think someone posted it here in Portuguese?).
Anyway, we terminated his contract, he became free agent. After that, we have no control in Joao Mario's next steps. He decided to sign for Benfica.
This is on Sporting themselves. They had agreed to pay us 7.5m€, they ended up offering less and I guess they offered less to Joao Mario too than initially agreed. Player got pissed, Inter got pissed. That deal went south, because of Sporting. Now they are just acting butthurt because they were the ones ruining the whole thing.