Hi Rakesh,
In the discount condition type, use calculation type "A"-Percentage, scale basis "C"-Quantity scale and scale type "B"-to scale. Maintain condition record with the to-scale quantity as 100 units.With these settings, irrespective of whether the condition update in the condition type is checked or unchecked, the system gives a discount of 20% upto 100 units of the material. For 101 units onwards, the discount value remains blank (discount does not apply automatically based on the condition record). The discount percentage can be manually entered in the sales order, if required.
I have tried these settings out and it works.
Regards,
Sumita Mittra