Johnson, a basketball analyst for ESPN, announced on the air Friday night he would write a check for $1 million to James as an incentive to take part in the high-flying exhibition next season.
"I'm going to put up a million dollars," Johnson declared. "One million dollars to LeBron, please get in the dunk contest. I go every year, I want to see you out there. A million to the winner."
While it wasn't clear if Magic meant James would cash in only if he was declared the winner of the contest, but SI.com said James would probably be the favorite.
James was presumably preparing Friday for Saturday night's big game against Memphis. The Heat have a 12-game winning streak going, but have lost twice to the Grizzlies this season, Sporting Alert.com said.
Toronto rookie Terrence Ross won this year's dunk contest, which James sat out once again despite his reputation as the best jammer in the league. His pre-game dunking exhibitions are a big favorite around the league, but James told SI.com his moves are more instinctive and not the pre-planned choreography seen in the all-start contest.
"I've always been an in-game dunker," James said last month. "When you've got to be in the dunk contest, you've got to be creative, figure out ways to do something that no one has done in the contest."