Hi @david @Schmitti ,
you’re right. XInvoice 3.0 does not allow to set a negative line item price, so you’ll need to set a positiv line item pricing with a negative quantity.
The feature for a “Stornorechnung” is missing, since a long time, since germany is probably one of a few countries that needs something like that 
.
I personally use credit notes, for cancelling a wrong invoice.
workflow:
- write an wrong invoice
- note that is actually wrong
- copy the invoice to a credit note
- apply the credit note to the invoice (both are then payed)
- create a new (hopefully now) correct invoice.
That’s the way the Finazamt and our tax adviser agreed on.