from www.tmz.com – Montana Fishburne claims she got screwed by her sex tape partner … over footage of some eXXXtra-curricular work they did on the side — and now she’s hired some lawyers to clean up the mess.

TMZ has obtained several letters that Fishburne’s legal team fired off to a porn star named Brian Pumper — demanding that his production company immediately stop all sales of “Phattys Rhymes & Dimes 14” … which features a scene between Montana and Brian.

According to the letters, Montana never signed a release for Brian to use the X-rated material in his video.

As for why Montana agreed to engage in coitus in front of Pumper’s cameras in the first place — Fishburne tells us Brian offered to help her “practice” for her big debut with Vivid Entertainment … and the footage wasn’t supposed to be sold.

So far, we’re told Pumper has not responded to the letters.