How to change discount tab & add new column in invoice

Hello All !!!

I am a hosiery goods dealer. We are using a define unit rate for products through out year (As we distribute Hard copy of price list to all customers in the beginning of new financial year) and as per the conditions we are changing discount on each item in fixed amount not in percentage.

Is there any option for change discount from percentage to fixed amount.

Also is there option to add new column in invoice

No, if you want to do fixed amount, add a line, use an Account called Discount Allowed or similar and enter a negative amount (-5.00)