Oktay, i think there's some kind of communication problem at our end. I didn't meant you to check it by doing posting block against your billing document type because you said that before creating accounting document you reversed the invoice so i just told you with that perspective as if accounting document hasn't been generated then there might be a posting block set for your return document type in VOFA (just an assumption).
Now coming to your issue go to VF02, give your billing document number click highlighted green flag and see what message you're getting is that the same mentioned in screen shot or different one? I'm afraid that accounting document has been generated against your billing document which has been overlooked.
If this isn't the case and accounting document hasn't been generated for your billing document but still condition tab is non-editable then check as if any enhancement has been made to set it in grey mode? I'm sure that i've made myself clear on this. In case you need further help, feel free to ask. Thanks.