Is it permissible for a non-Saudi imam to continue leading congregational prayers in the mosque and to receive an imam's salary from a Saudi individual who has been officially appointed as the mosque's imam, knowing that the non-Saudi imam is more deserving of leading the prayers and is better in recitation and tajweed?

It is not permissible for anyone to be appointed as an Imam for Awqaf mosques except for the one designated by the responsible authority. Therefore, it is not permissible for the person who will officially assume the Imamate to make you his substitute and give you the salary, except with official permission from the authority responsible for that. We advise you to seek lawful employment, and know that whoever gives up something for the sake of Allah, Allah will compensate him with something better.