Is it permissible for a person to act as a mediator between a service provider and a client, whereby he agrees with the service provider on a reduced price, then sells the service to the client at a higher price, with the difference being his profit?

If the client does not stipulate that the worker must perform the task himself, then it is permissible for the worker to hire someone else to do the work, as long as the work does not vary with different individuals, whether the payment is the same, less, or more.