Is it permissible for the union to demand an additional fourteenth salary, even though it is not stipulated in the employment contracts?

If one agrees to a specific salary in an employment contract, it is not permissible for them to violate the agreed-upon condition as long as the contract remains valid, based on the Almighty’s saying: "O you who have believed, fulfill [all] contracts," and the Prophet’s (peace be upon him) saying: "Muslims are bound by their conditions, except for a condition that makes a lawful thing unlawful or an unlawful thing lawful." It is not permissible to strike from work to pressure for a salary increase, nor is it permissible for a labor union to demand that an employee do something that violates the contract, such as striking. It is permissible to request a salary increase without violating the contract, but the company is not obligated to accept the request.