In a court filing Monday, lawyers for Rose Marks of Hollywood, Fla., and other members of her family said Deveraux loaned Charlie Stack $35,000 in July 2011, shortly after Stack's retirement from the Fort Lauderdale Police Department, the South Florida Sun Sentinel reported.
They say Stack used the money to start the RingFit Gym in Davie and quickly repaid it, around the time the Marks family was arrested in August 2011.
The Marks family faces federal charges of defrauding their victims of $40 million, including $20 million from Devereaux. Stack was a lead investigator in the case before his retirement.
Stack told the newspaper he expects to be cleared of any "improper relationship" with Deveraux.
"I'm a kickboxer, not an English major," he said in response to a defense charge that he helped the writer with her books.
Defense lawyers say Rose Marks was an assistant to Deveraux and earned any money she was paid. They say half the $20 million went to Deveraux's ex-husband in a settlement.
Deveraux, 65, has written dozens of romance novels, including 36 that have been New York Times best-sellers.