Syesha Mercado 'Book of Mormon' Star Sued for Sinning Against Manager

3/23/2014 12:57 AM PDT

Syesha Mercado -- 'Book of Mormon' Star Sued For Sinning Against Manager

Former "American Idol" star Syesha Mercado dropped her manager like a bad habit as soon she landed a big gig doing "The Book of Mormon" ... allegedly stiffing him out of hundreds of thousands of dollars.

Mercado -- who finished third on Season 7 -- is being sued by former manager Fred Crawford, who picked her up as a client not too long after her stint on "Idol."

Crawford claims Mercado agreed to fork over 20% of whatever she made in the entertainment industry.  Crawford says he helped her land the "Book of Mormon" deal ... for which she earned $40,000 a month.

But Crawford claims right after the ink dried Mercado cut off all communication and never paid him a cent. He's suing for his "Mormon" cut  -- roughly $300,000. 

Crawford has a big problem ... the deal, he says, was never committed to writing.

Calls to Mercado's rep were not returned, but we're guessing she tells a very different story.