What is the ruling on working for a financial brokerage company where clients deposit their money to make purchases from websites, knowing that some of these clients use the service to pay for poker gaming sites?

It is permissible to work with the aforementioned company in brokerage and benefit from the wage, unless one knows that the intention is to purchase something prohibited or to engage in gambling. In such cases, it is not permissible to deal with them, due to the Almighty's saying: "And do not cooperate in sin and aggression." The fact that the website deals with those involved in prohibited activities does not prohibit working in what is permissible and benefiting from the salary. However, it is obligatory to refrain from any prohibited transaction.