Hope this can help you .... I have coded a little function (apply_discount) in a inherited class of sale.order

def apply_discount(self, cr, uid, ids, discount_rate):
        cur_obj = self.pool.get('res.currency')
        res = {}
        line_obj = self.pool.get('sale.order.line')
        for order in self.browse(cr, uid, ids, context=None):

            for line in order.order_line:

                line_obj.write(cr, uid, [line.id], {'discount': discount_rate}, context=None)

        return res

I have added a new column to the new inherited sublass of sale order

'discount_rate' : fields.float('Discount rate'),

Then in the sale order view (a inherited one) I have placed the new field (discount) on the sale.order.view and I have fired an event on the "on_change" of the value passing the self value of the field to the "on_change" event

<field name="discount_rate" on_change="apply_discount(discount_rate)"/>

In this way you can apply discount sequentially to the rows of the order without altering the normal process of OpenERP

Hello

I have developed one module which set a discount on whole sale order and customer invoice with fixed and percentage vise. when your quote will move to invoice that same discount and total will move to the invoice I think this module feature will helps you more can see on : http://www.youtube.com/watch?v=RTRB0xq9Y38&feature=youtu.be To get the module can contact me: dsouzajoseph199@gmail.com

Our custom module for sale discount and invoice discount [Fixed & Percentage Discounts] supported For odoo10  & odoo9 .Even discount is passed to invoice when sale order is converted to invoice. 

https://github.com/euroblaze/eb_sales_invoices_discounts

Have you tried this - https://www.odoo.com/documentation/user/9.0/point_of_sale/advanced/manual_discount.html?

KM

Here are some approaches:

• You can try to use module additional_discount. But when reading the comments it seems that the module is not as stable as it should be.
• You can set the same discount in every line
• You can use a separate Discount product and give it a negative unit price. This solution would give an absolute discount instead of a relative.

You can use pricelists and the Prices Visible Discounts app (product_visible_discount).

You can use payment term, in payment term you can define your rule

In order to give discounts on the whole order, you have to use pricelists.

You can find them under: Sales -> Pricelists under the text 'Configuration' -> Pricelist

NB:

If you do not have 'Configuration', give the user the access rights 'Technical Features'

If you do not see Pricelist, give the user the access rights 'Sales Pricelists'

Now you can create a new pricelist, which you probably want to base on the default pricelist. I use in the title of the pricelist the amount of discount I give (5% discount, 10% discount, ....). Play around to find the correct value(s).

In the order it is important to change the pricelist, before adding any products, otherwise the discount is not calculated.

Important notice:

When using a discount like this, it will not show up on the discount line, but it will change the value of the product, so an article costing €10, and using a discount of 10% will show with a costprice of €9.

Complete Solution for Discount on sale tota, discount on invoice total, with report.

http://goo.gl/foA1pQ

and

http://goo.gl/YWw9JW