Discount amount in value not in %

Hi All,

     In customer invoice , we have option to give discount in %, Instead, Do we have option to key the discount value?

For eg:  Invoice amount value is 365.00 USD $, I like to give discount of 65 USD% . 

I want to get the invoice amount as 300 USD $ .

Please let me know where I can get this..  thanks in advance

Regads

Vijay