In sale order and inovice I need to calculate taxes before any discounts..and need to add some product to invoice as promotion ..with zero amount but taxes included.

Ex:

Amount = 100 £

Discount 1 : 10%

Amount : 90

Tax : 10% before discount = 10

Amount with tax = 100 

Pounce = 10 product = zero amount with 10% tax in base amount = 1£.

Total = 101 £

Im using odoo 10 CE..Any help?