We spoke with the 57-year-old -- who had the internet in a frenzy after he KO'ed a man while working a security guard shift in the UK earlier this month -- and he told us he's now willing to come out of retirement, if the opponent and the price are right.

"If there is a promoter that’s willing to promote a sanctioned fight," Julius said, "and, I never have liked talking about money, but [if] they could come up with a decent offer, then who knows?"

Francis told us he hears all the time that he should fight Jake or Logan Paul because of the potential huge payday ... but he doesn't view either of them in his category.

Instead, Francis shared there's only one person that could get him to lace up the gloves again ... and that's the man he lost to in a famous January 2000 bout -- Iron Mike.

"I’m going to put it out there," Francis said. "I’m going to say, 'Yes. Yes, we can have a rematch.' I don’t know if anybody will watch it, but I'm going to say, 'Yes.'"

Francis was unexpectedly thrust back into the spotlight while working a security gig outside of BOXPARK Wembley on June 12 ... when he knocked a man out with one punch after the guy had been agitating people nearby.

Julius said he felt "vindicated" by social media's response to his haymaker ... and that he was pleased to see people rally behind him, even going as far to say they appreciate what he did because the guy deserved it.

Francis expressed he has no regrets over the situation ... because he was simply defending himself against the guy.