Is the accountant obligated to inform the employer about incurring the costs of an employee who could have been dismissed, after the employer rejected the supervisor's advice to let him go and the employee completed his work, and is the accountant considered sinful, and if so, can he give charity on behalf of the employer?

Whoever seeks your advice, you should advise him with what you see as best and most suitable for the one seeking advice, because the consultant is entrusted. If you advise your brother on a matter when you know that rectitude lies elsewhere, you have betrayed him. So, the issue of sin relates to the extent of your sincerity in giving advice. You are not obligated to the employer at all, and the supervisor is responsible for any harm incurred by the employer due to the worker's remaining.