You don't renegotiate in the middle of contracts; this isn't the NFL. He was paid at or above market value for his second contract at the time he signed it and outperformed it. They rewarded him with a maxed out deal based on his previous contract.

Parker's value has nothing to do with Manu; and many people thought Parker's value was fair for what he brought to the team. I would say he outperformed it this past year and has a chance to outperform it for the next two if he remains the focal point of the offense.