Yes, I didn't know this but (according to the link @Minty put up the other day about engineering transfers) if a player doesn't put in a transfer request then the selling club have to pay up any "annual bonuses" in the contract up to the end of the contract.
I guess this means, whilst we may get more for the player, we actually have to fork more out to them when they go as well, swings and roundabouts I guess.