Fr. Ismael Moreno, known as Padre Melo, was friends with the Jesuit priests slain by SOA graduates in El Salvador in 1989.
The law firm's filing against Bishop Blase Cupich's diocese says the claims "are simply an attempt to throw mud [and] to try to get some insurance money."
The move is an attempt by the pontiff to address concerns that some accused clerics were not getting an adequate opportunity to defend themselves.
A Vatican court has "definitively determined and ruled" that no allegations of sexual misconduct of any kind alleged against Msgr. Richard Loomis, a Los Angeles priest, have been proved.
The ruling was announced in a statement issued Saturday by Office of the Vicar for Clergy of the Los Angeles archdiocese.
The ruling came "after 10 years of exhaustive investigation and canonical trial," it said, adding that Loomis "has always professed his innocence."
"We are committed to transparency with the people we serve. We cannot change the past but we hope we can rebuild trust through honest and open dialogue."
"You see the distance between the people and the church authorities; the people need to see in their authorities their own problems and lives."
Lawyers for the man, identified only as John Doe, said the archdiocese had provided false information in getting the man to agree to an $80,000 settlement in 2007.
Today, 77-year-old Fr. Gerardo Joannon lives in semi-seclusion, suspended from his parish work, accused of arranging illegal adoptions for teenage girls from prominent families.
Simply Spirit: Sometimes things are so breathtakingly awful, there is nothing to do but lament. Laments bring our brokenness to God
Nuns stand accused of working with a network of doctors, nurses and administrators to steal babies from mothers right after they were born.