What is the ruling on an employee remaining in a job where he receives a full salary from a company but is not assigned sufficient work, and is he sinful for this, especially since the matter is beyond his control?

A special employee is one whose benefit is measured by time, and who deserves the wage simply by making himself available to the employer during the hire period, even if he is not assigned any work. Therefore, there is no harm in taking the salary as long as you make yourself available during working hours.