Hi Waqas,

It is working really well. I thank you very much, it is a very good way to do what i want without any python line code ! 

One question : i need the product price to be on the invoice too. I modified the model of account.invoice.line and added also x_product_price,  related field product_id.lst_price like on sale.order.line. I also modified the views account.invoice.line.tree and account.invoice.line.form  like for the order. But on the invoice, i don't have the price of the product shown. Where am i wrong ?

My account.invoice.line.form is like this with the new field :

<?xml version="1.0"?>
<form string="Invoice Line">
                    <group>
                        <group>
                            <field name="product_id" context="parent and {'partner_id': parent.partner_id}"/>
                            <field name="x_product_price"/>
                            <label for="quantity"/>
                            <div>
                                <field name="quantity" class="oe_inline"/>
                                <field name="uom_id" class="oe_inline" groups="product.group_uom"/>
                            </div>
                            <field name="price_unit"/>
                            <field name="discount" groups="base.group_no_one"/>
                            <field name="currency_id" invisible="1"/>
                        </group>
                        <group>
                            <field domain="[('company_id', '=', parent.company_id)]" name="account_id" groups="account.group_account_user"/>
                            <field name="invoice_line_tax_ids" context="{'type':parent.get('type')}" domain="[('type_tax_use','!=','none'),('company_id', '=', parent.company_id)]" widget="many2many_tags" options="{'no_create': True}"/>
                            <field domain="[('company_id', '=', parent.company_id)]" name="account_analytic_id" groups="analytic.group_analytic_accounting"/>
                            <field name="company_id" groups="base.group_multi_company" readonly="1"/>
                        </group>
                    </group>
                    <label for="name"/>
                    <field name="name"/>
                </form>
            

Matz