Is it obligatory for an employee who has been negligent in his work but received his full salary to dispose of the books he bought with this money, given his intention to buy others with lawful money?

The employee must perform the required work to legitimize their salary. If they fall short without a legitimate Sharia (definition)-compliant excuse, the full salary does not become permissible for them. They must repent, avoid returning to negligence, and uphold trustworthiness. They are not required to dispose of the books they bought with the salary; rather, they can benefit from them. The portion of the salary corresponding to the deficiency must be disposed of by returning it to the employer if possible, otherwise by spending it on charitable causes. If the state provides an account for settling financial obligations, the money can be returned to it.