It's not interesting, it's complete nonsense, to the best of my knowledge, this is how add-ons work:
A team buys a player, pays a base fee and says the performance related bonuses will be paid out, if certain targets are met in the duration of the players initial contract with the team. If the player is then resold before his initial contract is up, the team will then have to pay all the performance related bonuses. Real Madrid had to pay Ajax around 6 million euros, when they sold him onto AC Milan, even though, he had not done enough, to meet those targets, but it has to be paid, if the player is sold on, during his initial contract.
It makes logical sense, because what's stopping a team from buying a promising youngster for a very small base fee, with the bulk of the fee in performance related bonuses, then just reselling him straight away before he can meet any of the targets?
So the theory that Tosic was benched as a money saving exercise is complete rubbish, because as his initial contract hadn't run out, we'd have defaulted on all his performance related bonuses that were owed to Partizan and would have had to pay them anyway when he was sold.