Jake Paul offered Tommy Fury a 'double or nothing' deal at their press conference on Thursday.
He suggested he would pay Fury double if he won their eight-round bout on Sunday. But if Fury loses, he would get nothing.
Fury rose and shook on it before the two were separated in a minor fracas.
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"You heard that, it's a deal," Paul declared.
Paul, the YouTube sensation turned prizefighter, is full of confidence he will win Sunday's contest against his first opponent with a professional boxing background.
"We picked this guy apart, we've studied him. He makes a lot of mistakes," Paul said. "He's a good boxer but he's not a great boxer and his lack of defence and lack of head movements is going to be exposed on Sunday.
"He looks like the Michelin Man or something. But there's nothing he can do to stop me. He doesn't have power," he added. "He can't knockout the taxi drivers he's been put in there with.
"He's never been put in there with someone who wasn't there to lose.
"He's never been eight rounds and he's going to get in the deep waters and drown. I promise that."
Fury fired back with his comments.
"This is in my heart, blood and soul and you're going to feel that," Fury said. "You've got a proper fighter in here and he's coming to take your head off.
"You should have stuck to easy money fighting old men."
He condemned Fury for boxing against mixed martial artists.
"MMA fighters are not boxers," Fury said. "I've absolutely took apart every single MMA fighter I've sparred with in my entire life.
"This guy couldn't even deal with them."
Paul sees his track record very differently.
"I've fought UFC champions. Tyron Woodley was winning the UFC championship while I was on the Disney channel," he said. "Anderson Silva's one of the greatest strikers ever.
"To go in there and outbox him and put him on the canvas, to me this is a speed bump and a cakewalk.
"I'm Tommy's end goal. This is the biggest opportunity of his life. He's never been on stage like this. He's never gotten a payday like this.
In contrast, he claimed: "I have more of a purpose.
"I want it more."