Will the head of the department be sinful if he lessens the tasks for collaborators who desire very little work, knowing that their contracts stipulate a specific number of hours and they receive a monthly salary for it?

You must assign the collaborators work that is appropriate for the agreed-upon time, which is two and a half hours daily, and that does not overburden them, while considering the interest of the work and not harming the workers. It is not permissible to show favoritism by assigning them work that takes less time than their pay, as this is negligence, harms the employer, and violates trustworthiness, unless the employer permits it. You are an agent for the employer, and an agent acts with permission and works for the benefit of his principal. He is not allowed to betray or waste public funds. The employee is obligated to perform the trust with honesty, sincerity, and care, to preserve time, and to fear Allah in his work, even if his supervisor is lenient.