McGregor has publicly called out Usman as he plots a summer return to fighting from a serious leg injury suffered in a defeat by Dustin Poirier last year.
The Irishman shared a training video as he steps up his intensity in preparation for a return, with McGregor striking pads and captioning the video: “I’m gonna break a head with these, add it to the list of heads.”
That drew a reply from Usman, who became welterweight champion in 2019, with the 34-year-old sending a jab both at McGregor and the former dual-weight champion’s teammate Dillon Danis.
“Even Danis looks ok on pads lol,” Usman commented.
That prompted McGregor to respond, in a tweet since deleted, with what appeared to be a shot at Usman’s inability to command the same level of financial reward for his fights that McGregor can.
Adding a laughing emoji, McGregor wrote: “Put your money where your mouth is then, oh wait.”
Usman reportedly earned in excess of $1million (£766,170) for beating Colby Covington in his last UFC fight in November.
By comparison, it was suggested that McGregor’s earnings from the Poirier defeat topped $5m (£3.61m).
Usman is expected to defend his welterweight title against Leon Edwards in his next fight.
Edwards is looking to become only the second British UFC champion.