Is it permissible for a worker to repeatedly ask for a tip despite not being given one?

The worker is not allowed to ask for payment for his work because he receives a wage for it from his employer. However, if the client rewards him voluntarily with a tip, and he is authorized to receive it by his employer—explicitly or by custom—then there is no harm in it. Otherwise, he is not permitted to benefit from what is given to him without referring to his employer.