Is it permissible for a former employee, who did not receive all his dues from the company, to compete with it over one of its clients with whom he built a good relationship during his employment, and what is the ruling on the company intentionally delaying the payment of dues?

Delaying a worker's wage is a forbidden injustice, as indicated by the Messenger of Allah (peace and blessings be upon him) who said: "Give the hired worker his wage before his sweat dries." The intention behind this command is to promptly pay the wage immediately after the completion of the work. As for leaving the company and working in the same field with its clients or others, there is no harm in that, as long as it does not involve making an offer over your brother's offer, or renting over your brother's rental.