The two drivers were summoned to NASCAR's at-track office on May 6 to discuss their ongoing feud, and NASCAR said afterward that the meeting "did not go as well as we had hoped it would."
Commenting on a meeting inside the hauler was unprecedented for NASCAR, which promised a further review of the incident. No fines were ever announced, and NASCAR said the issue was closed.
But rumors of the penalty began to leak on Monday in response to NASCAR fining owner Richard Childress $150,000 for attacking Kyle Busch after Saturday's trucks race at Kansas Speedway.
Newman, through a spokeswoman, declined to comment.
NASCAR spokesman Kerry Tharp also refused to comment on whether Newman was fined.
"Anything relevant to discussions that NASCAR has with competitors in the hauler will continue to stay between NASCAR and the competitors, and NASCAR will always work to protect that bridge of confidence," Tharp told The Associated Press.
But NASCAR has punished drivers for such incidents, putting Tony Stewart on a six-week probation after he punched Kurt Busch during a hauler meeting at Daytona in 2008.
If true, it's the second time NASCAR has fined Newman secretly. He was fined in April 2010 for comments he made about the style of racing at Talladega. It was not made public until the AP learned last July that Denny Hamlin had been secretly fined $50,000 for criticizing NASCAR on Twitter.
Newman then admitted he, too, had been fined.
NASCAR then faced a backlash for the secret fines, with fans complaining that privately punishing drivers was untrustworthy.
Since the Darlington meeting, both Newman and Montoya have declined to say what happened in the hauler.
"I don't know, ask him," Montoya said when asked if Newman had punched him. "I could tell you either way, couldn't I? I could make something up. Let's leave it at that."
Newman also refused to say whether the discussion turned physical.
"That's just speculation," he said. "It was a private meeting. That's why we had it at the hauler. With conflict there are varying opinions, that's what causes the conflict. I'm past it."