Hi Sibasish,
I have a idea:
1. You create a user defined field, it name Cash Discount. Value input to there is numeric.
2. Create a Refresh button, and write code to calculate for your formular: After you have entered price, % Cash Discount, you press Refresh Button, system will run your code.
3. Update field: Then, it will update value to Amount in Freight (I don't know can you do this, because this field in PCH3 Table)
4. Update Price if you want.
Then, add document if it right.
This is my idea, I have ever test this process before. Hope it right.
GBU