A retired cop who won a $3.9 million claim from the 9/11 victim compensation fund is being sued by a legal funding company for refusing to pay a $500,000 fee he agreed to for an advance on his payout.

New Jersey-based RD Legal Funding Partners is suing Elmer Santiago and his lawyers, the Manhattan firm Barasch & McGarry, which reps the thousands of WTC victims.

Santiago, who rushed to Ground Zero on Sept. 11, 2001 and worked a total of 200 hours on recovery efforts, suffers from respiratory diseases, according to court papers.

Santiago, 42, was advanced an initial $250,000 in August 2014 from RDLFP and bought a $282,000 house in Florida two days later. The Manhattan civil suit says he was supposed to pay the company back a total of $860,000 as soon as he received his money from the victim fund, but he has only paid a portion and still owes the $500,000 fee.

Victims are given 10 percent of their claims once they’re awarded from the fund. The rest is supposed to be paid out in a lump sum this year.

Santiago’s attorney, Michael Barasch, told The Post, that he plans to ask the Department of Justice whether the advance, which works out to a whopping 67 percent interest rate, violates criminal usury laws.

“I’m certainly not going to let Mr. Santiago be the victim twice,” Barasch said.