You traded people with guaranteed money still left on their contracts, whatever the bonus is for a player is money you MUST pay him, so if you trade him or re-negotiate there contract you still owe them the bonus (guaranteed) money...............
For example, Santonio Holmes just signed a 5yr 50M deal with the Jets, 25M was guaranteed, so if you trade him figuring you are saving his 10M per year, you are not, you save the 10M but you owe him all his bonus money still, so thats 25M, meaning you still lose 15M
Get it?