I think it's good to have a trap like this to weed out the bad managers who buy these type of players. It is a strategy game after all so it should be possible to make mistakes, even catastrophic ones.
That's a fine argument, and I'm not disagreeing with you here. There is a problem though, if you can buy a player like this specifically to win a single Tournament game, and sell him.
Tricking him out for a single game is not only currently possible, but the payment of salaries is such that you could buy a player like this on Tuesday, list him on Wednesday, he plays in your cup match on Thursday, and sells on Saturday
without you ever needing to pay his salary.