Where to file complain regarding deficiency in Services

I had an issue with NHBC where they deduct some extra amount on my home loan.

Earlier they are not reverting me anything when I complained to them, however when I raise a complaint on NHB portal they revert me back that it's their internal policy to deduct such extra interest which is also against the RBI and close the appeal/complain. In NHB portal there is no way to contradict to their point by replying or something. I do raise a complain with RBI ombudsman under deficiency in services but I got a reply that RBI ombudsman scheme does not work for deficiency in services cases. Can anyone please help suggest best way to deal with it legally.