According to new docs filed in the case -- which went to trial this week in L.A. -- Quincy says Sony Music and MJJ Productions took advantage of his advanced age. He's 84 now, and in the docs, says he was clearly over 65 at the time he sat down with the music execs to talk dough.
Quincy says Sony and MJJ pulled an accounting trick on him -- labeling a pot of money from MJ's music as profits, instead of royalties ... to which he would have been entitled a cut.
In short ... he's accusing them of bamboozling an old man.
Of course, Q produced MJ's 3 biggest albums -- "Off the Wall," "Thriller" and "Bad" -- and says the defendants have screwed him out of money earned off that amazing string of hits.
He's suing for at least $10 million, and is expected to testify in court this week.