It has been no secret that Floyd Mayweather's Aug. 26 return from a two-year retirement against UFC champion Conor McGregor has just about everything to do with money. 

What wasn't known until know is that Mayweather, who has earned approximately $700 million over his 19-year career, is in more need of the payday than you might think. 

Mayweather remains unable to pay a 2015 tax liability and filed a tax court petition on July 5 asking the IRS to allow him a reprieve until the McGregor fight. The filing also asks the government to waive a penalty for failing to pay.

"Although the taxpayer has substantial assets, those assets are restricted and primarily illiquid," the petition said, according to the legal website Law360. "The taxpayer has a significant liquidity event scheduled in about 60 days from which he intends to pay the balance of the 2015 tax liability due and outstanding."

According to a notice of a federal tax lien filed by the IRS, Mayweather owes $22.2 million in back taxes. Mayweather disputed the claim in a post Tuesday on social media. 

Both fighters addressed Mayweather's tax issues during Tuesday's news conference in Los Angeles, which marked the first of four consecutive stops on their international press tour. 

"The reason he has accepted this fight to come out of retirement is because he has to," McGregor said. "He's in a dire situation. That is not a good situation to be in."

Mayweather countered by telling reporters he "already took care" of his taxes and flashed an uncashed $100 million check written to him from his 2015 mega fight with Manny Pacquiao.

"I ain't worried about that. I just showed you all a $100 million check I didn't even cash onstage," Mayweather said. "So we ain't tripping on that."

According to the IRS website, the failure-to-pay penalty is typically 0.5 percent of owed taxes for each month. While Mayweather's 2015 taxes are 15 months past due, equaling 7.5 percent on top of what's owed. 

Mayweather (49-0, 26 knockouts) earned a guaranteed $100 million for his May 2015 victory over Pacquiao, and reportedly took home between $200 million and 300 million after pay-per-view receipts were counted (the fight sold a record 4.6 million buys). He returned four months later to defeat Andre Berto, earning a guaranteed purse of $32 million in his final bout before retirement. 

The fight against McGregor, which will take place in Las Vegas and will air on Showtime PPV, is expected to challenge financial and PPV records, despite the one-sided expectations. Mayweather, 40, will seek his 50th career win against a two-division MMA champion who has never boxed a professional round.