sale_discount_total/views/account_invoice_view.xml

<?xml version="1.0" encoding="UTF-8"?>
<openerp>
    <data>
        <record id="discount_view_invoice_line_tree" model="ir.ui.view">
            <field name="name">discount.account.invoice.line.tree</field>
            <field name="model">account.invoice.line</field>
            <field name="inherit_id" ref="account.view_invoice_line_tree"/>
            <field name="arch" type="xml">
                <xpath expr="//field[@name='discount']" position="attributes">
                    <attribute name="digits">(16, 2)</attribute>
                </xpath>
            </field>
        </record>

        <record id="discount_view_invoice_line_form" model="ir.ui.view">
            <field name="name">discount.account.invoice.line.form</field>
            <field name="model">account.invoice.line</field>
            <field name="inherit_id" ref="account.view_invoice_line_form"/>
            <field name="arch" type="xml">
                <xpath expr="//field[@name='discount']" position="attributes">
                    <attribute name="digits">(16, 2)</attribute>
                </xpath>
            </field>
        </record>

        <record id="discount_account_invoice_view_form1" model="ir.ui.view">
            <field name="name">discount.account.invoice</field>
            <field name="model">account.invoice</field>
            <field name="inherit_id" ref="account.invoice_form"/>
            <field name="arch" type="xml">
                <xpath expr="//field[@name='discount']" position="attributes">
                    <attribute name="digits">(16, 2)</attribute>
                </xpath>
                <xpath expr="//group[@class='oe_subtotal_footer oe_right']" position="replace">
                    <group col="4">
                        <group name="discount">
                            <field name="discount_type"/>
                            <field name="discount_rate"/>
                        </group>
                        <group class="oe_subtotal_footer oe_right">
                            <field name="amount_untaxed" widget="monetary" options="{'currency_field': 'currency_id'}"/>
                            <field name="amount_discount" widget='monetary' options="{'currency_field': 'currency_id'}"/>
                            <div>
                                <label for="amount_tax"/>
                                <button name="button_reset_taxes" states="draft,proforma2"
                                        string="(update)" class="oe_link oe_edit_only"
                                        type="object" help="Recompute taxes and total"/>
                            </div>
                            <field name="amount_tax" nolabel="1" widget="monetary" options="{'currency_field': 'currency_id'}"/>
                            <field name="amount_total" class="oe_subtotal_footer_separator" widget="monetary" options="{'currency_field': 'currency_id'}"/>
                            <field name="residual" groups="account.group_account_user" widget="monetary" options="{'currency_field': 'currency_id'}"/>
                            <field name="reconciled" invisible="1"/>
                        </group>
                    </group>
                </xpath>

            </field>
        </record>
    </data>
</openerp>