Mary's Briefcase is the briefcase full of money until Lloyd and Harry put IOUs in it.
Mary's briefcase is first shown in the movie as a part of Mary's luggage on her way to the airport. While on her way to her gate to Aspen, Colorado, she leaves her briefcase on the floor for Joe Mentalino and J.P. Shay to get so they can bring the suitcase full of money to their boss so that they can let her husband, Bobby Swanson go. But Lloyd, thinking that she left it on the floor by accident and forgot it, grabs it before the 2 kidnappers can and tries to return it to her. While they get to Aspen, Harry tries to throw it in the pond, but Lloyd stops him and they discover the money.