After Babylon 5 drove away the Shadows and Vorlons, Garibaldi resigned from his job because he was unhappy with Sheridan. Unlike Star Trek, money still matters quite a bit in Babylon 5.
Like any human on Earth today, a person needs to have lots of money to show the middle finger to his boss and quit without a job. Even if Garibaldi is rich, most of his assets would probably have been frozen or confiscated by the Earth government when Babylon 5 rebelled against Earth government. He is most likely poor after the rebellion.
How does he have the means to give Sheridan the middle finger and walk away without a job?