Is it permissible to agree with employees not to grant them end-of-service bonuses, even though the state law mandates them?

Labor law is considered an established custom unless the contract stipulates otherwise. In such a case, what was agreed upon in the contract takes precedence. If the contract does not mention an end-of-service bonus, then the labor law and its custom are to be referred to. However, if the contract states that the worker is not entitled to the bonus and he agrees to that, then he is not entitled to it, based on the Prophet's (peace be upon him) saying: "Muslims are bound by their conditions, except for a condition that forbids what is lawful or permits what is unlawful."