The legal situation is difficult, neither a), b) nor c) are an advantage for you. He probably charges roughly for service phases 1-3, depending on whether the 55k was meant with or without tax, a bit more or a bit less. Has he provided the services? He definitely created drafts. Basically, he owes effort, i.e. functioning drafts, but not necessarily success, which would consist in your satisfaction with the drafts. I would classify the invoice for now as a kind of defensive reaction, i.e. he wants you to continue working with him. If you definitely do not want to do that, in my opinion there are two possibilities. a) Transfer amount X (which you consider justified, e.g. 5000€) and write to him that in your opinion this corresponds to the effort made to draft to your satisfaction. He will accept the money 95% of the time and leave you alone, maybe he will go to court as well. b) Pay nothing at all and wait for a letter from the lawyer / court. The result here will also very likely be a settlement, but with significantly more effort / hassle involved. Do you really see no possibility at all to come to an agreement with the architect?