Is it permissible for an employee to take his employer's clients for his own private business at a lower price after the termination of his employment, and does the ruling differ if there was a prior agreement between the employee and the client?

It is not permissible for an employee to agree with customers to sell a product or service at a lower price outside of working hours, as this involves harming the employer and betraying trust. This prohibition includes agreements made during working hours. However, if a customer approaches the employee outside of their working hours and requests the same service that the employee provides at the shop, and the employee performs it for a lower fee, there is no harm in that, unless the employer stipulated in the contract that the employee should not work outside of working hours, especially if it affects their official work.