Is it permissible for an employee to take money from clients outside of the permitted cases to cover personal expenses without the company's knowledge, given that their salary is insufficient and their request for a raise has been denied?

What this young man is doing — taking a commission from clients outside the permissible circumstances and keeping it for himself — is not permissible. His low salary is not a justification for this forbidden act. He is only permitted to do what has been authorized by his employer. Even if he is given something by customers without him soliciting it, he must seek permission from his employer to keep it for himself; otherwise, he should give it to his employer.