Can a player ask his contract to be voided to make cap space or turn down full payment?

Example can amare say: Listen guys I cant contribute the way I want to so instead of paying me the 2 yrs 40 mil owed. i will only take 35Mil. use the othe r5 to go and get guy who can help us

No, Amare can't renegotiate:

http://www.cbafaq.com/salarycap.htm#Q59

59. Can existing contracts be renegotiated?

A contract for four or more seasons can be renegotiated after the third anniversary of its signing, extension, or previous renegotiation (if the previous negotiation increased any season's salary by more than 4.5%). Contracts for fewer than four seasons cannot be renegotiated. A contract cannot be renegotiated between March 1 and June 30 of any year.

Only teams under the cap can renegotiate a contract, and the salary in the then-current season can be increased only to the extent that the team has room under the cap (and cannot increase the player's salary beyond the maximum salary). A renegotiation can only be used to provide a salary increase -- players can't take a "pay cut" in order to create more cap room for the team.

If the player agrees to waive a portion of his trade bonus in order to facilitate a trade (see question number 97), his contract may not be renegotiated for six months following the trade.

Every category of compensation (base salary, likely bonuses, and unlikely bonuses) that are increased in the renegotiated season must also increase in all subsequent seasons of the contract. Raises (and decreases) in subsequent seasons are limited to 7.5% of the salary in the first renegotiated season.

A renegotiated contract can be extended simultaneously (see question number 58). If a player's contract is extended and renegotiated simultaneously in this manner, his salary may not decrease by more than 40% from the last season before the extension (after it is renegotiated) to the first season of the extension. For example, if the salary in the last season of a contract is renegotiated to $10 million and the contract is simultaneously extended, the salary in the first season of the extension cannot be less than $6 million.

Other rules for renegotiations:

A signing bonus cannot accompany a renegotiation unless the contract is extended simultaneously (see question number 58).

A rookie scale contract (see question number 48) cannot be renegotiated.

A contract cannot be renegotiated in conjunction with a trade.

You can renegotiate, but only for a pay increase, not decrease to the betterment of the team. Also, you have to be under the salary cap to even renegotiate, so the Knicks would have to move some contracts around first.