By giving back all the money she’s taken from him, Teddy is withdrawing her implied participation not only with the coming robbery, but with everything Louis did to earn that first wad of bills too. She’s making her distrust of him foremost in the relationship again. She’s choosing her self-worth over his expedience. And she’s doing it without being at all idealistic or naive: she knows damn well what that money could mean for her, but she knows she would betray herself by accepting it. She’s principled in a realistic, grounded way.
My long, long analysis of money and morality in Sharon Cullars' Filthy Lucre.