Is money earned from working in a joint-stock company, obtained after approval based on a request submitted in my name due to need, considered forbidden if it becomes clear that the company did not genuinely need me, and must I leave this job?

If you were appointed to the company without bribery or usurping anyone's right, and you fulfill your duties completely, then there is no problem with that. As for your misgivings about the manager nominating you without a real need, that is her concern, and you are only required to continue working according to the contract. The money you earn from it in return is lawful, provided that the field of work is permissible and does not entail anything forbidden.