Lindsay Lohan's Alleged Victim -- Pay Me At Least 100k ... Or Else

The man who claims Lindsay Lohan struck him while she was driving her Porsche last week is demanding "six figures" from her .... and she must pay before her big probation progress hearing.

Sources connected with alleged victim Thaer Kamal tell TMZ ... Mark Geragos -- Kamal's lawyer -- has contacted Lindsay's lawyer, Shawn Holley, and made the demand ... and Geragos made it clear that his client has not yet cooperated with cops by sitting down for an interview.

We're also told Geragos claims to have a surveillance video showing Lindsay behind the wheel as her Porsche strikes Kamal, then getting out of the car and wobbling into the passenger seat as the passenger takes the wheel and drives off. But our sources say none of the witnesses corroborate what Geragos claims is on video.

We're told Geragos gave Holley a deadline of today. That's significant because Lindsay is due in court on Thursday for her final probation progress hearing. If Kamal goes to cops and presents evidence of hit-and-run, Judge Stephanie Sautner could violate Lindsay's probation and she could go directly to jail.

On the other hand, if Lindsay pays the money ... Kamal could decide not to press charges and the case would be dropped.

By the way, we're told Kamal told people at the scene he was not injured but had a change of heart after learning the driver was Lindsay.