Sum in a Formular Column

Hi there, I set a board with a cost column and a status (to indicate if the invoice has been paid or not yet). The second cost column adds only the values of paid items with this formular:

IF(OR({Status}=“bezahlt”,{Status}=“beauftragt”),{Kosten inkl. MwSt.}," ")

So far so good, as long as all items in the group have the Status “bezahlt” = paid). If I add an item with an open amount, the Sum at the bottom of the group changes to “0”. How can I solve that?

Thanks a lot for hints and advices.

Update: it is different in browser and app as far as I see: in the app, the sum shows up, whereas in the browser, it doesn’t.