I can't see your screen shot, so it's like a walk in the dark.
I believe you can't reference the other column from your total column using inter record functions. But if you are using a partial sum and the Auftragsgruppe is unambiguous in that context, just call the field value:
=only(Auftragsgruppe) & ' Gesamt'
Now I can see the screenshot. The solution I suggested won't work.