Mickey Rourke's Opponent -- PISSED OVER PAYCHECK ... Where's My $5k?!

The man who "boxed" Mickey Rourke in Russia is PISSED -- telling TMZ Sports the actor's people are STIFFING him out of $5,000 he was promised for the fight.

We spoke with 29-year-old Elliot Seymour who says he was supposed to be paid $15k to step in the ring with the 62-year-old actor ... but only got $10k.

Seymour says the whole situation was sketchy -- but insists he was NOT paid to take a dive.

"People can say whatever they want ... if you've never been in a fight before in your life, you have no idea what it feels like to get hit."

Despite the financial dispute, Seymour says he's down to fight Mickey again -- if Mickey wants a rematch.

Rourke's boxing manager tells TMZ ... Seymour only got $10,000 because that's the most he can carry through customs. He says Seymour will get the other $5,000 next week, saying, "I think the poor kid is just enjoying the attention a little too much and is beginning to talk to the press about anything just to get more attention."