I haven't found an answer to this simple problem. So here is my simple solution. (Some translations might be a better solution. But just for one country (e.g. Germany) it should work.)

Edit the QWeb report report_invoice_document.

Settings -> Reports (Berichte) / Invoices (Rechnungen) / QWeb Views (Suche nach verknüpften QWeb Ansichten) / report_invoice_document

There is the following to edit: 

<span t-if="o.type == 'out_refund'">e.g. Stornorechnung</span>

In V8 there is a module "cancel invoice".

After installation you can tick a checkbox in the sales journal to make a "storno". it enables to set the invoice back to DRAFT.

Servus Tobi

Hi,

Thanks but thats not what Im looking for. I already kinda have the possibility to reset an invoice, but what I need is something similar to a credit memo. In Germany we have both, ordinary credit memos ("Gutschrift") and something similar to that called "Stornorechnung". I don't know the exact differences besides a different text on the document, a different sign (uncertain about that) and maybe another account for the bookkeeping.

But still thanks alot for the try Tobias!

Regards Sascha