R. KELLY’s child pornography case is coming back to haunt him – he’s been accused of failing to pay a private investigator he hired to recover evidence that could have incriminated him.

Private detective Charles Freeman is suing Kelly for outstanding fees totaling $300,000.

The overdue amount is alleged to relate to Kelly’s 2008 trial, in which he was exonerated of 14 charges, including having sex with a minor and videotaping himself engaging in sexual intercourse with a girl as young as 13 years old.

Kelly, who faced up to 15 years in jail, has always maintained his innocence.

However, in legal papers filed last week in Missouri federal court, Freeman alleges Kelly hired his services during the well-publicized trial to “obtain and/or recover certain tapes that were said to incriminate” the singer.

Freeman is seeking damages plus interest and costs.