Lindsay Lohan's got more than fiance trouble ... a filmmaker is going after her for the $10,000 he claims he gave her without getting a damn thing in return.
Matt Martin made a movie called "Little Mermaid," not the Disney one, and approached a Lindsay associate about having her pump the flick to her nearly 5 million Instagram followers.
Matt says in the legal letter, obtained by TMZ, he struck a deal with Yossi Bechor back in October after learning he often arranged deals on Lindsay's behalf. He says the deal was $10k and executive producer credit ... for LiLo to post 3 promos for his film, but 3 months rolled by without a peep.
Martin says he's reached out to team Lindsay and Yossi several times to no avail. So now he's demanding he get the dough back, or he'll sue.
We reached out to Lindsay and Yossi, so far no word back.