Since October 2020 the option of partial refunds in Admin UI is available. Partial refunds can be made, like normal refunds, by clicking the refund button. What changes is that you can now edit the amount of the refund. This should be only done when a Vendor asks to initiate a partial refund.
If the whole amount should be refunded, do NOT change anything.
Process:
Check the gross price of the product
Check if the vendor agreed with the partial refund and insert the amount that should be refunded
Make sure refund is not bigger than paid amount
Initiate refund
The buyer will get the refund, the reverse billing (Stornorechnung) and the new invoice with the right amount automatically.