Overeem said on The MMA Hour that he still hasn't been paid for winning the K-1 World Grand Prix Final in 2010.
"They didn't pay me," Overeem said. "It's a lot of money, I can tell you that."
Overeem said he expects to take legal action against FEG, the parent company of K-1, but he's skeptical that he'll ever collect what he's owed.
"We're on the verge of starting up legal action, but Japan is a hard environment to move. You have the language barrier, you have the culture barrier. Are we expecting something from it? It's hard to say. I actually think it's going to be really hard. FEG has a lot of outstanding liabilities, a lot of unpaid bills, so maybe mine is just going to be put underneath the stack of the other bills. They have a lot of debt."
Overeem said he feels even worse for Peter Aerts, whom Overeem knocked out to win the K-1 World Grand Prix, and who didn't get paid anything.
"You get your ass kicked and you don't get paid? That's just a shame," Overeem said.
But Overeem said he's pleased with his recent business decision to sign with the UFC, where he will face Brock Lesnar in December. Overeem said that he was engaged in tough negotiations with Strikeforce, which is also owned by UFC parent company Zuffa, before finally being released by Strikeforce only to be re-signed by the UFC. He says that was a real negotiation and that it wasn't just posturing by the UFC to move him from Strikeforce to the UFC.
"I don't think it all was a trick and a ploy to get me into the UFC," Overeem said.
Now Overeem's focus is on Lesnar, whom Overeem views as a dangerous opponent.
"I do definitely think he brings it. I do definitely think he fights to win, to destroy his opponents," Overeem said. "I expect him to come full force. I expect him to be 100 percent come Dec. 30."
If Overeem beats Lesnar, he would likely face the winning of the upcoming heavyweight title fight between Cain Velasquez and Junior dos Santos.
"There are a couple of guys who are interesting fights for me and those guys happen to be in the UFC," Overeem said. "That is the big show these days. There were other fights out there, there were other options, but everybody has been sending me messages, hassling me, telling me go to the UFC, Alistair, when are you going to the UFC? ... I always want to fight the best guys, the best guys are in the UFC, so of course I want to fight in the UFC."