Michael Jackson's Pianist Conrad Murray Screwed Me Out of 'This Is It' Riches

3/3/2014 12:15 AM PST

Michael Jackson's pianist could have been huge -- hauling in nearly half a million bucks -- if Conrad Murray didn't kill MJ ... so now he's threatening to sue Murray for all the money he lost.

Morris Pleasure was in the band for the "This Is It" tour, and last fall -- while Conrad was still in jail -- he fired off a letter accusing the doctor of robbing him out of what would have been his "highest-paying job."

Pleasure's no stiff -- he's performed with Earth, Wind & Fire, Christina Aguilera, Janet Jackson, and Natalie Cole, Roberta Flack and many more -- but the MJ gig would have been his pinnacle of his career.

We're told Mo P stood to make about $400,000 over 2 years of touring with Michael.

By law, Murray had 90 days to work out a settlement -- but the piano man's attorney tells us they've heard nothing ... so the next step will likely be a lawsuit for the lost wages.