Is it permissible for an employee to work only one hour a day at his job, after he became able to complete eight hours of work in one hour due to experience, while his manager refuses to increase his salary or alter the initial agreement?

If the agreement between the employee and the company is for a fixed salary in exchange for a specific number of working hours (8 hours), then what is considered is the completion of the full working hours, not the quantity of production. The employee is not obliged to continue with them unless the contract is for a specific period. Otherwise, he has the option after completing a month or a year; he may choose to continue or request a raise. If he does not find another job, he must fulfill the first contract.