The only way is years.
It depends on how long Pierce thinks he will play in the NBA, how long the C's think they will play.
They would have incentive to get the next years salary lower given a the tax hit, which they *might* not have later, and given the salary cap could go up. They declining deal potentially helps them later.
I could see him going down to 15 range for a four year deal, maybe with a club option for the fifth so they can call it a 75 million dollar extension. There is a good chance he won't be able to make more than the MLE anywhere in two years, and I just think he won't want to be a FA next year (since he has a high salary and a team that could win it all right here.)
I don't think the club has a huge incentive to do anything though.