What is the ruling on working an administrative job with the title "Purchasing Representative" in a residency, knowing that this administrative profession is "Saudized"?

If the work assigned to you is something that is overlooked and customarily permitted due to necessity, and it does not contradict the general purpose of the prohibition, then there is no harm in it being different from your job title. However, if it is not permitted at all, then you must adhere to what is specified in the employment regulations, because the ruler has the authority to restrict what is permissible if there is a public interest in doing so.