Tony Azzopardi, 56, had told the Daily Mirror earlier this week that Winehouse approached him outside a pub in Camden, north London, on the evening before she was found dead at her home. He claimed the singer paid a dealer £1,200 ($2,000) for half an ounce of crack and the same amount of heroin.
Azzopardi was questioned for 30 minutes at Holborn Police Station on Tuesday, the Metro freesheet reported Wednesday.
On leaving the station, he was restrained by officers after hurling a stream of abuse at the waiting reporters, the newspaper added.
London's Metropolitan Police said no arrests linked to Winehouse's death were made.
Police were now analyzing surveillance footage around Winehouse's home in a bid to find out more about her movements, the Daily Mirror reported Wednesday, as her family believes the "Black to Black" star stayed in on the night of her death.
Also, despite Azzopardi's claims, police said they found no evidence of drugs or drug paraphernalia in her Camden home.
Winehouse was found dead by her bodyguard at around 4:00pm local time on July 23.
Azzopardi had said he wanted to help the police work out what happened to the singer in her last hours, but claimed he was terrified of reprisals from the dealer.
"I want Amy's family to know the truth about what happened," he said. "I want to help them out -- but I'm worried about my safety."
He added that Winehouse complained to him that her ex-husband, Blake Fielder-Civil, had called her from prison and been "hassling" her.
Her father, Mitch Winehouse, 60, told mourners at the star's funeral that she quit drinking after being clean of drugs for several years. The family believes she died of a seizure or heart attack because her body could not handle going "cold turkey" from alcohol.
An autopsy proved inconclusive, and it could be several weeks before the results of toxicology tests which may establish the cause of her death.