Is money earned from salary during an absence from work without submitting medical certificates permissible or forbidden?

If the management systems allow absence for the mentioned period, then there is no harm in it, and the salary is permissible (halal). Medical certificates can be provided later. However, if the systems do not permit this, and it is not stipulated in the contract, then it is not permissible, and the salary is unlawful (haram) and must be returned to the management. The permission of the department head is considered valid if it falls within his authority; otherwise, it is a betrayal of the work.