Justin Bieber: Sean Kingston's A Liar -- I Didn't Gift Him My Car!

Sean Kingston is LYING through his pearly white teeth ... 'cause Justin Bieber DID NOT "gift" him his customized Fisker Karma. Kingston began the charade in front of Boa this weekend ... when he rolled up to the steak joint in JB's  chromed-out car and told photogs Justin had given it to him as a gift ... as a perk of their friendship. But sources close to the The Biebs says Kingston's pants are COMPLETELY on fire -- truth is, Sean asked Justin if he could BORROW the car to go to dinner since Bieber is on tour ... and Justin obliged. Now, the car is back in Justin's garage -- and we're guessing Kingston's driving privileges have been suspended indefinitely. Kingston's people say he was just joking when he said JB gave him the whip.