In the pilot of Ozark, we learn that Marty's partner Bruce scammed high ranked cartel member Del and stole 8 million dollars.
Del spares Marty's life in exchange of the retribution of the 8 million, and Marty laundering for him.
In 48 hours, Marty manages somehow to get the hand on the 8 million (probably in Bruce's secret account?), and simply withdraw the cash from the bank like it was his own personal and legal money to hand it to Del.
How did he do that?