Well, I guess you could manage everything in FI/CO, credit management etc. in spreadsheets or other similar software, you can also pay for additional tools and systems to create and process country-specific documents like NFe, which among other things depend on having a correct accounting document generated from SD billing.
However disconnecting SD from other modules somehow defeats the purpose to pay for SAP licenses and hire consultants to configure and manage the system - you can easily create nice forms for calculating prices and print nice documents with FOSS. 