Is it permissible to work in the morning without the employer's permission, given a contractual clause that prohibits working for another party, and provided that the morning work does not affect the evening work?

The condition stipulated by the employer, which prohibits working for another entity, must be fulfilled, based on Allah'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 Haram thing Halal or a Halal thing Haram." This is also because the default rule for contracts and conditions is permissibility and validity. Furthermore, this condition serves a purpose for the employer, ensuring the worker's ability to perform their job efficiently. Therefore, it is not permissible to engage in another job without the employer's permission.