In a unanimous decision, Quebec's Court of Appeal panel ruled that officers cannot be forced to provide documents that could incriminate them to the watchdog.
The charges against the former right-hand man to Gérald Tremblay, who was mayor from 2001 to 2012, allege that municipal contracts were awarded in exchange for political donations.
The province's highest court has ordered that the case against Jacques Delisle return to Quebec Superior Court, where he had been facing a second trial.