HOW TO CHANG THE TYPE OF TOTAL DISCOUNT FROM PERCENTAGE TO AMOUNT IN SALES_ORDER
It is a customization, not available with standard.
For one of our customers, we implemented a new field to input “Total discount amount” and we converted that value into “Total discount percentage” field. That reduces customization efforts. (you should take care of rounding problems).
in invoice avaliable or no
No, It’s not available.