Is the monthly bonus still a right for employees, knowing that it is disbursed with the signature of the financial manager without the knowledge of the new general manager, and it was previously approved by the authorized board member?

If it is within the managing director's authority to disburse the bonus, then there is no objection to taking it. However, now the financial manager must inform the general manager about it, and the bonus is not permissible for you unless the general manager approves it. The fundamental principle is that money is inviolable, and only what is taken with willing consent is permissible. It is not permissible to take it through deception or fraud, as Allah Almighty says: ﴿O you who have believed, do not consume one another’s properties wrongfully, but only [in lawful] trade by mutual consent from you﴾ [An-Nisa: 29], and as the Prophet, peace and blessings be upon him, said: "Indeed, your blood, your properties, and your honor are sacred among you," and "The property of a Muslim is not lawful except with his willing consent."