7:02 PM PT -- Fanmio CEO Solomon Engel is firing back at Diaz ... calling his lawsuit "frivolous" and claiming the fighter has already received a seven-figure check for the event in the process.

"I look forward to resolving this dispute through the appropriate process and am confident that justice will prevail," Engel said in a statement obtained by TMZ Sports.

"Making salacious and defamatory statements to the media in order to harm my family and I has only strengthened my resolve to ensure that the truth will triumph."

Nate Diaz says he was promised $10 million for his fight with Jorge Masvidal ... but he claims he never got 90% of the cash -- and now, he's suing to get money he says he rightly deserves!

The former UFC star's attorney, Jeremiah Reynolds, filed the lawsuit in U.S. District Court in Miami on Monday ... less than two weeks after he and Masvidal squared off in a boxing match at the Honda Center in Anaheim.

Diaz claims in the suit, obtained by TMZ Sports, that before he ever stepped in the ring with Game Bred ... he made a deal with promoter Solomon Engel and his company, Fanmio, in which he agreed to take the bout in exchange for a $9 million payday.

But Diaz says after he beat Masvidal on July 6 in a 10-round clash ... Engel refused to pay up. According to Diaz, Engel said he couldn't supply the funds because he didn't profit off the fight like he expected.

The lawsuit also claims Engel told Diaz he couldn't pay him, despite verbal and written agreements, because his wife might divorce him over the financial loss.

Diaz is suing for fraud and breach of contract ... stating in the documents that he would have never have gotten in the ring with Masvidal if he knew he wasn't getting paid.