Narendra,
From my understanding, the requirement is
How can we Block Sales Order ,if due to Qty Scales ,Price of any Line Item becomes Zero.
It might be a possibility that the business user creates a sales order with any qty (e.g. qty below 100).
So it should be possible for the user to create a sales order with any qty. But in case of less than 100 qty, the subsequent documents should not be allowed to be created.