'The Purge: Anarchy' Actor -- I Got Robbed ... They Never Said I'd Be on Billboards

"The Purge: Anarchy" actor featured on billboards across the country says producers hoodwinked him ... because no one said he'd be the face of their ad campaign -- and now he wants to be paid.

Emmanuel Howell says he worked 5 days on the shoot as an extra and made less than a grand. He expected to only be in the background of shots ... but months later, he started seeing his mug pop up on billboards.

Howell tells us he reached out to production asking for more dough -- but they said to kick rocks ... because he should be happy for the honor. Instead, they offered him free digital movie posters.

Howell says he contacted multiple lawyers because it felt like a slap in the face ... but so far, they've all said he doesn't have a case.

The billboard star explained his entire situation to our photog ... and you can't help but feel bad.

Welcome to Hollywood!