What is the ruling on taking money that a tenant insists on giving to a worker in a property without the employer's knowledge, despite the worker's refusal of it?

It is not permissible to accept a commission or a gift from the buyer without the employer's permission, because "gifts to workers are ill-gotten gains" (ghulul), and because it is an increase in price that belongs to the employer, and it is considered a type of prohibited bribery.