**Impacted versions:**
* 9.0

**Steps to reproduce:**
* Enable multi currencies
* Make sure that the current rate of a foreign currency (i.e. EUR) is not 1.0 (i.e. 0.84)
* Create a purchasable product with a cost (i.e. 33$)
* Create a vendor bill in a foreign currency (i.e. EUR)
* Add the product on the invoice

**Current behavior:**
* The unit price will be 23.28 == (33 * 0.84) * 0.84

**Expected behavior:**
* The unit price should be 27,72 == 33 * 0.84
* Probably due to this line of code: https://github.com/odoo/odoo/blob/9.0/addons/account/models/account_invoice.py#L1241

**Video/Screenshot link (optional):**
* On Runbot Community 9.0: https://www.youtube.com/watch?v=60qPmczo5vk


addons/account/models/account_invoice.py
def get_view_id(xid, name):
            try:
                return self.env.ref('account.' + xid)
            except ValueError:
                view = self.env['ir.ui.view'].search([('name', '=', name)], limit=1)
                if not view:
                    return False
                return view.id
        context = self._context
        if context.get('active_model') == 'res.partner' and context.get('active_ids'):
            partner = self.env['res.partner'].browse(context['active_ids'])[0]
            if not view_type:
                view_id = get_view_id('invoice_tree', 'account.invoice.tree')
                view_type = 'tree'
            elif view_type == 'form':
                if partner.supplier and not partner.customer:
                    view_id = get_view_id('invoice_supplier_form', 'account.invoice.supplier.form').id
                elif partner.customer and not partner.supplier:
                    view_id = get_view_id('invoice_form', 'account.invoice.form').id
        return super(AccountInvoice, self).fields_view_get(view_id=view_id, view_type=view_type, toolbar=toolbar, submenu=submenu)
    @api.multi
    def invoice_print(self):
        """ Print the invoice and mark it as sent, so that we can see more
            easily the next step of the workflow
        """
        self.ensure_one()
        self.sent = True
        return self.env['report'].get_action(self, 'account.report_invoice')
    @api.multi
    def action_invoice_sent(self):