Is the preparation of fictitious accounts and fabricated budgets for a company, with the aim of obtaining a periodic classification certificate, permissible or forbidden, knowing that they are merely accounts on paper and do not result in any material gain for anyone?

What you have mentioned about forging accounts contrary to reality in order to obtain a classification certificate is not permissible, because it involves forbidden deception and fraud. This is based on Allah's saying: "So avoid the word of falsehood," and the Prophet, peace and blessings be upon him, said: "Whoever cheats is not one of us," and he also said: "Beware, and I warn you about false speech and false testimony." Forgery includes deception in documents and records with the intent to mislead and lie. It involves deception and manipulation to gain an undeserved classification certificate. And whoever claims what he has not been given is like one who wears two garments of falsehood. Therefore, it is not permissible to work in that.