Greenwood's performance was slated to cap a daylong tribute to the military, police and firefighters.

But Jerry Bentley, Greenwood's manager, told the Denver Post that concert organizer Frank Young failed to pay the singer $20,000 in cash as he had promised.

However, Young said he had $18,000 in cash but Greenwood's handlers refused to accept a $2,000 check from the Knights of Columbus for the balance, the Post said.

Many of the 850 fans who turned out for the event were furious that Greenwood, famous for the song "God Bless the U.S.A.," walked out without performing.

Young took the stage and broke down in tears as he explained what happened, said witness Jeff Alumbaugh.

"He told everyone, if you want your money back, we'll give it to you, and the entire crowd said 'no,' which says a lot about the crowd," Alumbaugh told the Post.

"We all just looked at each other and said, 'This is the patriot, the man who's made millions off of us being patriotic?" said fan Kia Ritland, adding the crowd joined hands and sang "God Bless America."