Floyd Mayweather says he has not agreed to face Tenshin Nasukawa in an official bout and had "never heard" of the Japanese kickboxer until this week.
The former five-weight boxing world champion was said to have agreed to face Nasukawa in Japan on 31 December.
Mayweather, 41, says he had only been asked to compete in a non-televised event for "wealthy spectators".
"Once I arrived to the press conference, my team and I were completely derailed," said Mayweather.
"We should have put a stop to it immediately.
"I can assure you that I too was completely blindsided by the arrangements that were being made without my consent nor approval. I was hesitant to create a huge disturbance by combating what was being said and for that I am truly sorry."
The unbeaten American extended his record to 50 wins in August of 2017 with victory over UFC star Conor McGregor in a bout which earned him a reported $300m (£228m).
It was unclear under what rules a one-off bout with Nasukawa, 20, would take place. Nasukawa holds a record of 27-0 in kickboxing and a 4-0 in Mixed Martial Arts.
Mayweather continued: "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.
"I was asked to participate in a nine-minute exhibition of three rounds with an opponent selected by the Rizen Fighting Federation."
In an extensive Instagram post detailing his version of events, Mayweather did not make clear if the exhibition contest will still go ahead.
In July, he was named the world's highest-paid celebrity by Forbes, beating actor George Clooney into second place.