Discount at Order Header Level

Hi,

For a specific customer, my users want to give a lumpsum discount on the total order amount.

If I create a modifer and set the level as "Order", it allows only % discount to be given on the order. There is no way to give an absolute amount.

Moreover the amount deducted reflects on the sales order line as well.

Is their a way to give discount on the total order amount without impacting the lines?

Thanks in advance,

Jaya