bonus comes into effect is if the player retires like Aaron Rodgers was threatening to do. Had he actually retired, he would have had to pay back the prorated portion of his signing bonus because he did not honor the contract.

Teams prorate signing bonuses over the length of the contract for salary cap purposes, so it is generally written into the contract that if a player does not complete the contract, they have to give back the prorated portion of their signing bonus.

On the other hand if the team trades the player they just eat that signing bonus cost, but that's their choice. They don't have to trade the player. They're making a choice to do so, and they are generally getting back value that they think is worth making the trade either in other players, or draft picks.

And if they decide to release the player, they are still on the hook for the value of that signing bonus prorated against the salary cap. That is when you get into what is called dead money.