Liability of service tax on service receiver

Dear All

Please suggest your view in the following Circumstances-

Whether the Service receiver can deny payment of Service Tax to the service Provider on the Ground that when originaly the Contract entered between two Party the category of service to be provided was cenvatable and later on due to change in statutory Rules the category of service become non-cenvatable? Here both the Service Provider and Service Receiver are Corporate.

what is the Sanctity of Such clause in the Contract?