Sgt. John Wasilenko, of the Elmwood Park police department, cashed the check written on the account of Angelo Ditore in December 2012, WBBM-TV, Chicago, reported Friday.
Wasilenko initially went to Ditore's apartment in October 2012 after Ditore reported $70,000 missing. No one was arrested in that incident.
Ditore is now in a nursing home, a ward of the state and the Cook County Public Guardian, Robert Harris.
Harris is suing Wasilenko for financially exploiting the elderly man.
"We know there was a check written for $20,000, and then there's some talk about some other monies," Harris said.
Wasilenko told police investigators the check was a "gift" from Ditore for running errands.
It's against department policy for officers to receive any kind of gratuity from citizens, said Elmwood Park Police Chief Frank Fagiano.
Wasilenko was placed on administrative leave last week after the department learned of Harris' suit.
He was suspended in September 2012 for allegedly using police resources to harass and run background checks on three men.
Fagiano said he gave Wasilenko his job back at the same rank after that incident because he didn't want to jeopardize criminal investigations.