A promise is a promise, even in nine-figure territory.

Heavyweight boxing champion Wladimir Klitschko told the Dan Patrick Show that legendary boxing promoter Don King once offered to pay him and his brother Vitali $100 million to fight each other in the ring, according to Larry Brown Sports.

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The brothers both were heavyweight champions and held different belts simultaneously. The 38-year-old Wladimir said he thought Vitali, 43, who retired to become a politician, would win if the two ever fought.

Wladimir told the Dan Patrick Show that the two sometimes sparred when they were starting their boxing careers, but they promised their mother that they would never fight each other in the ring.

So when King brought up the suggestion, the younger Klitschko said he didn't believe him.

"He said, 'You guys sign with me, I'm going to make this fight. I'm going to make you rich,' Wladimir said, laughing.

Wladimir revealed that whenever the two would spar, both became emotional during the sessions. The last time they were toe to toe in the ring - when Wladimir was a teenager - he broke his leg.

I don't know what happened," Wladimir said. "I was 16, and I broke my leg. And since that, we just decided not to do it anymore. I think it's a sign from above, 'Just don't, guys, fight anymore.' It's so emotional because none of us wants to give up. Each of us wanted to try to show that, 'I'm stronger and better than you.'

"Don't ask me why or how. I didn't kick. It wasn't a kickboxing competition or MMA or something like that. For some reason, it just happened."

Vitali, which Larry Brown Sports reported retired with a 45-2 record, now is the mayor of Kiev. Wladimir is 63-3 and is scheduled to fight in April.

He and fiancée Hayden Panettiere just had a baby girl.