Of course their are laws but that does not mean it’s impossible. It used to be common practice both in Spain and France. There was always a small room set aside in the notary’s office for the counting of the cash paid ‘sous la table’.
The difficulty is it requires the agreement, trust and co-operation of all parties involved.
Recently the Hacidenda have been cracking down. An unconvincing low ‘paper’ selling price may well attract attention.
I would not advise doing it in today’s climate.