While Zugferd allows for "regular" invoices with negative amounts, an invoice item can not be negative, we get the validation errors
'The Item net price shall NOT be negative' and 'The Item gross price shall NOT be negative'.

What they do allow is negative quantities, so an item with a price of -100€ (e.g. a "gutschrift") can be submitted as "-1x 100€", with the invoice total being a regular "-100€".