The Nevada State Athletic Commission (NSAC) claims that Floyd Mayweather Jr. didn't follow the rules when he sought approval for an intravenous drip before his victory over Manny Pacquiao.
The US Anti-Doping Agency (USADA) gave permission to Mayweather to use the drip of saline and vitamins on the eve of his 2 May fight after the fact, but the NSAC says that only it could approve the drip and that USADA had no authority to do so.
As such, Pacquio is insisting on a rematch, according to AFP.
USADA claims that it gave Mayweather a Therapeutic Use Exemption (TUE) for the drip in accordance with NSAC rules.
NSAC executive director Bob Bennett, however, dismissed this notion.
"Anyone should know worldwide, if you fight in the state of Nevada we are the sole authority for approving a TUE," he said in an interview with the BBC.
"USADA has been known historically as the premier performance-enhancing drugs-testing organisation and they have an outstanding record. But they do not have the authority to authorise a TUE to a fighter in the state of Nevada."
The fact that Mayweather had received the infusion was already a point of controversy, since such a drip is normally administered to combat dehydration after a tough training session.
Mayweather, though, dismissed suggestions that he had received favourable treatment from USADA.
"I follow and have always followed the rules of Nevada and USADA, the gold standard of drug-testing," he said.
"Let's not forget that I was the one six years ago who insisted on elevating the level of drug-testing for all my fights."
Bennett added that it is highly irregular for a fighter to be granted a TUE on the eve of the fight, but stopped short from saying that Mayweather had received preferential treatment.
"The process is you go online and you submit an application with supportive evidence from your doctor. Upon our receiving it, we give the application to our doctor. After his review we will determine if a TUE is approved," Bennett continued.
"We are very meticulous, we look at each TUE very thoroughly before our doctors make a final decision. We want to do all this for the health and safety of the fighter.
"I don't recall a time when we have authorised a pre-fight TUE for an intravenous drip. I'm not saying it hasn't ever happened, but I don't recall it."
According to BBC correspondent Ade Adedoyin, who is based in Las Vegas, the Pacquiao camp is "quite unhappy" with the situation.
"One of his business advisors has spoken to the media and has been saying how ironic it is for Mayweather to be in this situation now," Adedoyin said.
"He has also says that perhaps the best way forward is for Mayweather and Pacquiao to have a rematch."
Part of the reason Pacquiao insists on a rematch is because he was refused an anti-inflammatory shoulder injection before the fight, the richest in sports history – after failing to alert the authorities in time.
After his defeat, he blamed his shoulder injury for the defeat.
"That is why I want a rematch," he is quoted as saying by AFP.
"One without any injury and with fair play. No favouritism. Not one where the Mayweather camp gets to dictate all the terms and conditions.
"The Mayweather camp accused me of using performance-enhancing drugs. Now look what happened. The truth has finally came out and I was vindicated.
"If needed, the NSAC should impose the appropriate sanction to sustain its credibility and to show the world they did not give preferential treatment to the Mayweather camp."