Hello, I can suggest one third party module 

(https://apps.odoo.com/apps/modules/14.0/discount_account_invoice/). 

I was used this module earlier. So by this module, we can give discounts as Fixed discounts or Percent discounts in Invoices and Bills. The discounts can be applied on the Invoice lines/Bill lines or globally on the entire Invoice /Bill. The discount can be applied on taxed or untaxed amount in the invoices and bills.

Try this, hope it will helps you..