Kevin Durant is calling B.S. on The Game -- telling TMZ Sports he never agreed to write a $20,000 check if the rapper hit an NBA-distance 3-pointer.

The controversy stems from a post Game put on Instagram on Wednesday -- in which he wrote, "Bet Kevin Durant $20,000 that he'd have to donate to the @froggallstars & play wit #LaFamilia in the #DrewLeague if I made an NBA 3 pointer all net."

Game says several celebrities -- including DeMar DeRozan, James Harden & Drake -- were on hand to witness the wager.