Rizin Fighting Federation announced a bout between Mayweather and Nasukawa for New Year's Eve earlier this week, with the two fighters appearing alongside one another at a media event in Tokyo.
Yet boxer Mayweather, now back in the United States, says he never agreed to fight Nasukawa and understood that he would have an exhibition bout against an unnamed Rizin fighter for "a very large fee".
The 41-year-old, who boasts a perfect 50-0 record, explained he did not want to let down fans who travelled from around the world to be in Japan by causing a scene at the promotional news conference and insists he is still retired.
"Now that I am back on US soil after a long and disappointing trip to Tokyo, I now have the time to address you, my fans and the media, in regards to the upcoming event on December 31 that was recently announced," Mayweather wrote on Instagram.
"First and foremost, I want it to be clear that I, Floyd Mayweather, never agreed to an official bout with Tenshin Nasukawa. In fact (with all due respect) I have never heard of him until this recent trip to Japan.
"Ultimately, I was asked to participate in a nine-minute exhibition of three rounds with an opponent selected by the Rizin Fighting Federation. What I was originally informed of by Brent Johnson of One Entertainment was that this was to be an exhibition put on for a small group of wealthy spectators for a very large fee.
"This exhibition was previously arranged as a 'special bout', purely for entertainment purposes with no intentions of being represented as an official fight card nor televised worldwide.
"Once I arrived to the press conference, my team and I were completely derailed by the new direction this event was going and we should have put a stop to it immediately.
"I want to sincerely apologise to my fans for the very misleading information that was announced during this press conference and I can assure you that I too was completely blindsided by the arrangements that were being made without my consent nor approval.
"For the sake of the several fans and attendees that flew in from all parts of the world to attend this past press conference, I was hesitant to create a huge disturbance by combating what was being said and for that I am truly sorry.
"I am a retired boxer that earns an unprecedented amount of money, globally, for appearances, speaking engagements and occasional small exhibitions."
Mayweather has recently talked up the possibility of facing UFC star Khabib Nurmagomedov, having defeated rival Conor McGregor in the ring last year.
UFC president Dana White insisted any clash between Mayweather and Nurmagomedov would have to take place in the octagon, with the proposed contest against Nasukawa seen as a potential first mixed martial arts fight for the American.
Photo: Sean M. Haffey/Getty Images Caption: Floyd Mayweather Jr