1. There must be a net price list.
This can be done via Settings -> Price lists.
This price list can be named something like 'business customer' and of course needs to be net.
2. The article to be sold need to have a price in the newly created business customer price list.
This can be done via Master data -> Articles -> Search.
If there is no price in this price list, it will falls back to the UVP which is not a net price list and is shown in the invoice as including taxes.
3. The customer needs to be connected to the business customer price list.
Price lists can be connected via Master data -> Customer -> Search. After clicking on Edit, you can select the correct price list.