What is the ruling on money that a father brings from his work in customs, knowing that he requests it from container owners?

Theft is not the only forbidden means of earning money. Gifts given to employees because of their job are considered bribery, betrayal, and consuming wealth unlawfully, especially if the employee requests them. The questioner and his mother are correct in deeming these things forbidden, and they should advise their father to avoid them and not accept them.