You can give the sincere and hard working workmen with incentives / productivity bonus for every month for which you must maintain the productivity of each workmen. Even you can pay as per the quality products delivered by the workmen, for which no one can ask any question. But take prior approval from your management before effecting the payment. This payment you need not to give through the contractor or his bill. You can directly make the payment (Cash Reward) to the workmen in monthly progress /quality / safety meeting in front of all the employees.