It is very sensitive issue. By outsourcing the labour what did you gain? You have to pay the contractor based on his payment of wages which should be at least at par with the statutory minimum wages fixed by the government. You have to compensate the EPF and ESI payments made by the contractor. You have to ensure that these contributions are paid by the Contractor every month. You should also reimburse the bonus paid to them. At the end of the day, if a dispute arises, you have to look in to it. If you have engaged them in core activities, then the labour authorities will question it and they can even abolish such engagement. If the arrangement with the contractor is seen to be sham, then you will have to absorb every one under your own rolls. Therefore, what is the real benefit that you get by such engagement? Nothing, I would say.
At the same time, if the contract is genuine and contractor is taking care of everything, including the service book of the labourers then there is no risk in such deployment. However, there should not be any document to establish that you have direct supervision over these workers. If there exists a system that you bargain with the workers for fixing their wages, that they take your permission for taking leave, etc, then the situation will lead to make the contract sham and that may lead to serious issues. Very simple, it all depends on how do you manage the workforce of the contractor. As far as possible, ask the contractor not to send the same person to your plant regularly but to send different persons.