Is my work considered unlawful (haram), given that my job requires me to prepare invoices and accounts that include an increase in the prices of goods (theft), approved by the employer, knowing that I only receive my monthly salary and do not take any unlawful (haram) money?

If you are an accountant responsible for financial matters and monitoring their flow in the company, you must advise the deceiver and cheat to fear Allah and repent. If he desists, then praise be to Allah. Otherwise, you must inform the relevant official. If that official is also involved, then inform those above him. It is not permissible for you to remain silent about this evil. If you are unable to do so, then look for another job. It is not permissible for you to remain in this job unless there is an urgent necessity.