Floyd Mayweather’s motivation to climb back in the ring and fight UFC champion Conor McGregor may have been revealed. The undefeated former world champion says he can’t pay off his tax liability from 2015 until he fights the Irishman in August.

The Law360 website reports that according to a tax petition filed on 5 July, the 40-year-old has asked the IRS to delay his payments until he picks up his purse from August’s fight. “Although the taxpayer has substantial assets, those assets are restricted and primarily illiquid,” the petition said. “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.”

It is not known how much Mayweather owes but he settled a $6.2m lien he had from 2007 after fighting Juan Manuel Marquez in 2009. He is estimated to have earned $220m from his fight against Manny Pacquiao in 2015, and similar sums have been discussed for the McGregor bout. Mayweather has never been shy of discussing his spending. He has been known to spend $1m betting on single sporting events, and his collection of watches is worth $6.4m alone.

The promotional tour for the fight starts this week in Los Angeles, and will also take in New York, Toronto and London.

In a hype video released this week, McGregor said he expected to win easily: “I’m going to be ruthless in there,” McGregor says. “My fist is just going to break his whole face and that’s it.”

Mayweather has won all 49 of his professional fights, and is a five-division world champion. McGregor has never boxed professionally.