Is it permissible to keep an amount that was mistakenly added to the salary, especially since the salary is low and does not reflect the effort expended, and previous notifications to the company have been ineffective?

You are not permitted to keep money to which you have no right. It must be returned to the company or their permission obtained, because Allah has forbidden consuming people's wealth unjustly, commanded the fulfillment of trusts, and prohibited treachery. The questioner's mention of achieving great profit for the company is a requirement of his work, and a low salary is addressed by demanding an increase or leaving the job.