New Price on Order Header

Hello All

There is a requirement where we need to adjust the order amount to match the customer PO.

So suppose PO is of amount ABC  and entering the sales order with the items it comes to amount 'XYZ' from the pricelist, we need to adjust the Order header amount to 'ABC'

Now when i user a Discount modifier on the header, it only allows 'percent' as the application method. To do this manual calculation is required to get the percent. Is there a way we can adjust the header amount directly like a 'new price' without calculating the percent or any other better way to adjust the price.