Is it permissible to work with the company's clients after leaving it, in violation of the contract which stipulates not to work with them for 5 years, and is the money earned from such work unlawful, and what should I do if I have already started working and received salaries?

The company does not have the right to prevent you from working with any other entity after the expiration of the contract between you, even if the one you will work with was a former client of the company. The company only has the right to prevent you from disclosing trade secrets, based on the Almighty's saying: "O you who have believed, do not betray Allah and the Messenger or betray your trusts while you know [the consequence]."

The condition that you do not work with any of the company's clients after the expiration of your contract is a void condition, based on the Prophet's (peace be upon him) saying: "Every condition that is not in the Book of Allah is void," and his saying: "Muslims are bound by their conditions, except for a condition that prohibits what is lawful or permits what is unlawful."

This condition prohibits what is lawful, which is a person working with whomever they wish after the contract ends without causing harm. It involves undue restriction and limitation on the worker without justification, and it causes harm unjustly, based on the Prophet's (peace be upon him) saying: "There should be neither harming nor reciprocating harm."

What you have earned from this work is not forbidden to you if it was permissible.