Is the difference in salary considered a right of the petitioner based on the employer's promises or not, and is it permissible for him to take it without the employer's knowledge?

The validity of a contract lies in what was verbally agreed upon, even if it was not written, because Allah Almighty said: "O you who have believed, fulfill [all] contracts," and because the Prophet, peace and blessings be upon him, said: "Muslims are bound by their conditions."

Fulfilling a promise is recommended according to the majority of scholars. If the employer promised the employee a raise, and the employee continued working based on this promise, then fulfilling the promise here is obligatory.

The employee has the right to refuse to renew the contract upon the expiration of the agreed-upon term. The issue of entitlement to something due to the promise, or lack thereof, is to be referred to the judiciary or presented to scholars.