The guy who claims Britney Spears' hit song "Piece of Me" jacked sexy moans from his sound library ... is backing down because the lawsuit was taking a huge piece of his wallet.

Kirk Rothum voluntarily dismissed the suit against Sony Music. You'll recall, Rothum went after Brit's label back in May, claiming the seductive "uhh" and "oh yeah" moans were ripped, without permission, from his sound library. Rothum says he never got a dime.

A source close to the case tells us Rothum decided to wave the white flag because he was tired of wasting money without the suit going anywhere.

We're told there's still a slight chance he could refile, but until Rothum's got money to burn -- looks like Sony's off the hook.