Why would Roger Mason Jr. accept a negotiated buy out? If he's not willing to accept a buy out, to leave money on the table, why would the Knicks release him? They waived Azubuike instead, a completely useless player (even more useless than Mason).
Brewer requested a buy out because D'Antoni told him he had no plans of using him and NY agreed to save some money and as a favor to Brewer's agent.
Roger Mason makes $1.4M this year, barely above the league minimum, and he's expiring. New York could have just waived Roger Mason and it hardly would have made a difference.
I don't know how muc $$ Brewer left on the table with his buyout, but he was making ~$3.7M this year, also expiring. Figuring that Brewer probably didn't walk away from THAT much money (he probably walked away from whatever the pro-rata vet min is for the last 20 or so games, so maybe a couple hundred thousand). I don't think NY saved a lot of money by buying Brewer out instead.
I don't get D'Antoni. Just don't get him. Brewer's a legit rotation player, if you're serious about making a playoff push, why wouldn't you want him around to help defend Pierce/Allen and LeBron/Wade?