Is it permissible or impermissible for an employee to continue working in a slaughterhouse where there is deception and theft of meat, with the knowledge of the officials, and to receive a wage for that? And is the salary received by the employee during the period in which he was trying to rectify the situation considered permissible?

You have done well in denouncing the evil. If you have done what you can, then there is no sin upon you, for Allah Almighty says: "O you who have believed, عليكم أنفسكم [take care of yourselves]. No harm will come to you from one who goes astray when you have been guided." Your salary is lawful for you and is not affected by the cheating and injustice of others, as long as you do not assist them or approve of their actions.