The E-Invoicing Act of 4 April 2017 (‘Act on the Implementation of Directive 2014/55/EU on Electronic Invoicing in Public Procurement’) supplemented the E-Government Act (‘Act on the Promotion of Electronic Administration’) with regulations on e-invoicing (cf. esp. Section 4a EGovG). With the E-Rechnungsverordnung (E-RechV) of 6 September 2017, further regulations on the design of electronic invoicing were issued by the Federal Government.
Background information on the legislation can be found here:
In the German federal states, electronic invoice exchange is regulated independently: The corresponding implementation acts define when an acceptance obligation for public authorities exists and whether suppliers/service providers have an invoicing obligation.
KoSIT summarises the implementation status of the German federal states and makes it available.