Marty Singer -- the Tuohy family attorney -- said it all happened recently ... and, what's worse, he claimed this is not the first time Oher has done this.

Singer -- in a lengthy statement to TMZ Sports -- said Oher came to the Tuohys prior to filing his 14-page petition in Shelby County, Tenn. ... and threatened them, saying if they didn't pony up an eight-figure check, he'd "plant a negative story about them in the press."

Singer denied all of the allegations in Oher's court filing ... explaining that Sean and Leigh Anne Tuohy absolutely did not trick the former NFL star into getting into a conservatorship when he was 18 years old as he had claimed.

The conservatorship "was established to assist with Mr. Oher’s needs, ranging from getting him health insurance and obtaining a driver’s license to helping with college admissions," Singer said. "Should Mr. Oher wish to terminate the conservatorship, either now or at anytime in the future, the Tuohys will never oppose it in any way."

As for Oher's claims that the Tuohys used the conservatorship to broker a movie deal with Fox behind Oher's back that netted them millions -- while leaving Oher with nothing -- Singer said that was BS as well.

"The notion that a couple worth hundreds of millions of dollars would connive to withhold a few thousand dollars in profit participation payments from anyone -- let alone from someone they loved as a son -- defies belief," Singer said.

Instead, the attorney claimed Tuesday the Tuohys had "received a small advance from the production company and a tiny percentage of net profits" from "The Blind Side" ... money Singer said the Tuohys have always either shared with Oher or have tried to share with the former Baltimore Ravens tackle.

"Over the years, the Tuohys have given Mr. Oher an equal cut of every penny received from 'The Blind Side,'" Singer said. "Even recently, when Mr. Oher started to threaten them about what he would do unless they paid him an eight-figure windfall, and, as part of that shakedown effort refused to cash the small profit checks from the Tuohys, they still deposited Mr. Oher’s equal share into a trust account they set up for his son."

Singer continued, "Unbeknownst to the public, Mr. Oher has actually attempted to run this play several times before – but it seems that numerous other lawyers stopped representing him once they saw the evidence and learned the truth. Sadly, Mr. Oher has finally found a willing enabler and filed this ludicrous lawsuit as a cynical attempt to drum up attention in the middle of his latest book tour."

Singer added the Tuohys are "heartbroken" over it all and they hope to reconcile with Oher soon ... though "they will not hesitate to defend their good names, stand up to this shakedown and defeat this offensive lawsuit."