Pat Riley opened his mouth ... and now he's gotta open up his wallet.
The Heat prez has been hit with a BIG fine for comments made about a potential reunion with LeBron James in Miami ... with the NBA determining he broke the league's anti-tampering rule.
Riley -- who orchestrated the Heat's Big 3 of LeBron, Dwyane Wade and Chris Bosh that won 2 titles in 4 years -- recently opened up about James in an interview with Dan Le Batard .