Advance pricing on credit order from History

Hi,

I want to apply the discounts on the credit order from history.

1)Create credit order

2)Use Credit Memo to retrieve the Original SO.

3)Select the order

4)Press OK and credit order gets created

5)Now go in the detail of Credit order and press Price history or check price.

6)I am not able to enter an extra discount on the credit order.

Why when going in the price history I cannot put the information.Attaching the screen shot.

Thanks