Da Brat -- I'm Paying Off My Debt ... Slowly, But Surely

Da Brat is already made a dent in paying off her massive $6.4 million debt ... $400 down, $6,399,600 to go.

The rapper performed Saturday night at a bar in Atlanta ... and according to Dennis Byron -- the guy who shot the video -- Brat's raps were inspired by her prison stint.

We broke the story ... Brat has to pay an Atlanta Falcons cheerleader $6.4 million for beating her face in with a bottle back in '07.