Is it permissible for an employee to sell materials obtained in exchange for their leave balance, and are the profits generated from selling these materials lawful?

Selling these materials to customers is permissible due to the absence of stock within the company. As for taking the difference between the customer's selling price and the employee's selling price, it is permissible if the employee is aware of this and gives permission for it; otherwise, it is not permissible. If the enquirer is merely an agent for the employee, then he must give the employee the full price. However, if the employee wants a specific amount, and anything in excess of that is for the enquirer, then there is no harm in taking it, and the relationship would be considered an agency for a fee.