EXCLUSIVE
Da Brat is already paying off her massive $6.4 million debt ... by $400.
The rapper performed Saturday night at a bar in Atlanta ... and according to Dennis Byron -- the guy who shot the video -- Brat's rapping about prison and the case she's involved in.
We broke the story ... Brat has to pay an Atlanta Falcons cheerleader $6.4 million for beating her face in back in '07.
We're told Da Brat got paid for the gig ... but based on the setup, it couldn't have been much.
