It is amazing to think about these numbers as a "discount" but they are much lower than the max he could have gotten.  Marc Stein reports:

Salary figures obtained by ESPN.com show that Garnett's salary will be $14.7 million in the 2009-10 season, $17.1 million in 2010-11 and $19.5 million in 2011-12, when he'll be 36. Garnett was already scheduled to earn $22 million and $23 million over the next two seasons.

(For salary-cap purposes, however, his salary in each of those years will jump by $1.75 million to account for the trade kicker, making it $23.75 million for 2007-08, $24.75 million for 2008-09, $16.4 million for 2009-10, $18.8 million for 2010-11 and $21.2 million for 2011-12.)