Remember, the NCAA launched a massive investigation into claims Manziel was making dough signing memorabilia (a no-no for college athletes) while in Miami for the 2013 BCS National Championship game ... but found zero evidence of the QB getting paid.

Johnny Football is now coming clean about the whole ordeal ... breaking down the entire thing with Barstool Sports' "Bussin' With The Boys" podcast.

"This guy comes up behind me, he's like, 'Yo, how would you wanna make 3 grand?'" Johnny said. "I turn around, I'm like, f*** yeah, bro!'"

Manziel said he had about $65 in his bank account at the time and wanted some extra cash ... so he followed the guy to his condo to sign "probably 10,000 pieces."

But, it didn't stop there -- Johnny says another dude approached him and said he was getting royally ripped off (which, uh, yeah, he definitely was) ... and offered him a sweeter deal -- $30k!!!

"So, this guy's pretty much, 'All right, go to this room at the Fontainebleau. All this stuff will be in there laid out. When you're done, just send me a picture of all of it. I'll give you the code to the safe, the money will be in there,'" Johnny says.

Pretty sketchy, right?? But, Johnny says he didn't care ... 'cause he was finally making some money ... and has no regrets over the serious NCAA violations.

In fact, he says he went back several more times over the next few months to get some more coin.

"We went 9-4 the next year, so if the NCAA wants to take my f***ing 9-4 season away, my Chick-Fil-A Bowl against Duke, f***ing blow me," Manziel said.

So ... does this mean Manziel's Heisman could get taken away like Reggie Bush's?? Johnny's adamant he didn't break any rules until AFTER he won the trophy.

"I never took a dollar until after I won the Heisman, and I think my statute of limitations are up. So, you can blow me again!"