Undefeated boxer Floyd Mayweather goes by the nickname "Money" but a lack of cash prompted him to file an appeal with the IRS regarding overdue taxes.

Mayweather filed a tax court petition on July 5, asking the IRS to grant him a reprieve and allow him to pay his taxes after he is paid for next month's bout vs. Mixed martial arts star Conor McGregor.

The 40-year-old Mayweather is coming out of retirement to face McGregor on Aug. 26 that could net the boxer a nine-figure payday.

The IRS wants Mayweather (49-0) to pay his taxes for 2015. Mayweather has earned an estimated $700 million in his career, Forbes magazine reported, including a staggering $220 million from his much-hyped fight against Manny Pacquiao in May 2015.

"Although the taxpayer has substantial assets, those assets are restricted and primarily illiquid," the petition by Mayweather said in a filing posting on Law360.com.

"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 the IRS website, the penalty for failure to pay taxes is typically 0.5 percent of owed taxes for each month.

Mayweather retired from professional boxing after defeating Andre Berto in September 2015. McGregor (21-3), who has never boxed professionally, defeated Eddie Alvarez at UFC 205 in November 2016.