Sale of exempted goods with duty

in DI Pipe plant, zinc wire is used as input and zinc dust is generated out of the manufacturing process of DIP. This zinc dust is exempted from levy of duty vide circular Circular No. 1027/15/2016-CX dt. 25.04.2016. We have one opinion that we can sell this with duty as customer has no objection if we levy the duty. However, another opinion is that since this is exempted goods, duty cannot be levied and reversal under rule 6(3) is only option. Since calculation under rule 6(3) is very critical, the preferred way would be to levy duty on sale. is this correct?