A freelance designer who says his work was stolen by an advertising agency working on the Hollywood remake of cult thriller Oldboy has written an open letter to director Spike Lee asking him to intervene.

Juan Luis Garcia says posters based on his designs are being used to promote the film, which is released this weekend in the US, despite the fact that he has not been paid for his work or agreed to their use. He says the unnamed agency involved made an "insultingly low offer" when it decided to use his designs, and continued to use them when he declined their offer.

"I make the same amount of money in a single day as a photo assistant as what they offered, and I had worked on these almost exclusively for two months," writes Garcia in the letter. "We never signed any contracts or work-for-hire agreements and I certainly never agreed to donating or selling any copyright of my work without a licensing fee. I never even got paid the peanuts they owed me [for the original design pitches]."

Garcia says he was threatened with legal action when he complained about the agency's actions. He made the decision to bring the matter to Lee's attention when the film-maker began posting the designs, which feature star Josh Brolin, on his social media pages.

"I couldn't believe that you had been using and claiming copyright on three of those very same posters I designed," he writes. "I just couldn't believe it. I perceive you as an advocate of the arts and artists and have a sinking feeling that you are as much of a victim in this as I am."

Garcia added: "I know you'll understand my story of an artist trying to make a dignified living. It's difficult and sometimes seems impossible because everyone wants you to work for free or for 'exposure'."

Interviewed by the Hollywood Reporter, the designer said his letter was a final attempt to resolve the issue prior to legal action. "I don't want to sue anyone, it's not in my nature, but if that's what it comes down to, so be it," he said. "I'm thrilled he liked the posters and hope they continue using them, but I need to be remunerated."

The new version of Oldboy, loosely adapted from South Korean director Park Chan-wook's blistering revenge tale, has so far met with lukewarm reviews and is predicted to perform weakly at the box office. It centres on the release of a kidnapped man held prisoner in solitary confinement for 20 years.

Wrote the Guardian's Tom Shone: "The timing of this film seems both unfortunate and salutary. In this banner year for black film-makers, taking on subjects as great as slavery and the civil rights movement, here comes Spike Lee, without whose example none of their films would exist in quite the form they do, and what has he got for us? Some karate moves, fancy camerawork, and a wink-wink cameo for a cephalopod."

Lee has not yet made any public comment on Luis Garcia's letter.