Stallone didn't say why Willis had left the project, but he did describe him as "greedy and lazy" in a tweet.
The Hollywood Reporter said Willis was offered $3 million for four days of work shooting in Bulgaria.
"He said he'd drop out unless he got $4 million," a source close to the production told the entertainment industry trade newspaper.
"A million dollars a day. Stallone and everybody else involved said no," the insider said. "I think [Willis] was pretty surprised he was replaced in 72 hours by Harrison Ford -- a better actor, a much nicer person and a more interesting direction for the film."
The sequel is to begin shooting this month.