Concert promoter Kenneth Carey just filed suit against DB, his entertainment company and a bunch of the goons that were with him that day ... alleging the rapper not only illegally battered him but also turned his back on a deal they'd struck to host an event.

Long story short ... Kenneth now claims in legal docs -- obtained by TMZ -- that he and DaBaby agreed to have the latter host a party where another artist, Stunna 4 Vegas, had been booked to perform at. Ken says he and DB agreed on a flat rate ... $20k, no more.

In his lawsuit ... Ken says when DaBaby and co. arrived in town, he arranged hotels and cars for them via a rental company -- and alleges one of those vehicles was returned damaged.

Fast-forward to the day Ken was supposed to pay DB, and he claims the guy asked him for an additional $10k on the spot to cover the damage to the car, which apparently got billed to DaBaby and his crew. Ken says DaBaby insisted it was the least Ken could do ... seeing how he agreed to do the hosting gig for way cheaper than he normally charges.

Ken says he refused to pay another $10k, and that's when the alleged ass-whooping -- part of which was caught on camera -- went down. The promoter says DaBaby and at least 4 other guys pulled his pants down, beat him up and poured apple juice all over him.

He also claims the crew jacked him of his phone, money and credit card that he had on hand. Adding insult to injury, Ken says neither DaBaby or Stunna showed for the event. The suit was filed by Jonathan May of The Lions' Den law firm in Miami.

Of course, DaBaby was later arrested and charged for battery in connection to this case ... but he later sprung himself from jail and defended himself online, denying the account.