What is the ruling on an employee taking their rights themselves, such as extra leave or overtime pay, if the company did not raise their salary after the currency flotation? And is it permissible for them to do so with their manager's approval?

You do not have the right to take money from the company. However, you are permitted to ask your employer for a salary increase. If they do not grant it, you may leave the job after the contract expires. If you agree to stay, you are not permitted to violate the terms of the contract, for Allah Almighty says: "O you who have believed, fulfill [your] contracts," and the Prophet, peace and blessings be upon him, said: "Muslims are bound by their conditions."