Kamaru -- the reigning UFC welterweight champ -- told TMZ Sports back in April he was getting sick of Paul's trash talk and said he would love the opportunity to "beat the s**t" out of him.
Paul responded to Usman, "Challenge accepted" -- adding, "If your boss Dana gives you permission to box me and make more money than you ever have, let me know and we can lace them up. I promise it will be your biggest pay day."
The two continued to trade shots over social media -- with Jake accusing Usman of "backing out."
Which led to Usman responding yet again on Wednesday morning ...
"I understand you think this is a game which is probably why you don’t realize that this is how people Truly get hurt," Usman said.
I understand you think this is a game which is probably why you don’t realize that this is how people Truly get hurt. I don’t play fighting and I can change your life in the worst way. https://t.co/Zch2xlDA9b