Is a person sinful if they take an amount exceeding their salary, where the accountant made a mistake in their calculation, and the accountant told them, "There is no problem with that, take the amount, as the company intended it and I cannot take the money from you"?

You must return the amount you took to the company, because the accountant is not authorized to exempt you from returning it. The duty of trustworthiness necessitates returning it to the company's treasury. If an authorized person pardons you, then it is permissible for you to own the money; otherwise, you must return it.