Is my work in a restaurant that serves imported meat mixed with local fat, selling it as local meat, unlawful (haram)? Am I sinful because of this work? And what should I do with the money earned from it?

Mixing local fats with imported meat and selling it as local meat is forbidden deception, because the Prophet, peace and blessings be upon him, said: "Whoever deceives is not from me." Your work in preparing this deceptive meat is impermissible because it is assisting in sin, and you must refrain from it and leave your job at the restaurant if it necessitates deception. If you repent, the salary you received is permissible for you. You must advise the restaurant owner to fear Allah and stop deceiving, and that he owes financial rights to those he deceived.