Hi,

You have to use onchange for this:-

        The "onchange" mechanism provides a way for the client interface to update a form whenever the user has filled in a value in a field, without saving anything to the database.

Here there is already onchange for product_id so:-

To achieve this you have to override product_id_change() in sale.order.line .

For this you have to inherit sale.order.line object and redefine the product_id_change() by adding this line:- (after the line defining product_obj )

     result['discount'] = product_obj.x_discount 

For more help:- if you are using v7 copy this code in your module .py file:-

from openerp.tools import DEFAULT_SERVER_DATE_FORMAT
    
class sale_order_line(osv.osv):
    _inherit = 'sale.order.line'
    
    def product_id_change(self, cr, uid, ids, pricelist, product, qty=0,
            uom=False, qty_uos=0, uos=False, name='', partner_id=False,
            lang=False, update_tax=True, date_order=False, packaging=False, fiscal_position=False, flag=False, context=None):
        context = context or {}
        lang = lang or context.get('lang',False)
        if not  partner_id:
            raise osv.except_osv(_('No Customer Defined!'), _('Before choosing a product,\n select a customer in the sales form.'))
        warning = {}
        product_uom_obj = self.pool.get('product.uom')
        partner_obj = self.pool.get('res.partner')
        product_obj = self.pool.get('product.product')
        context = {'lang': lang, 'partner_id': partner_id}
        if partner_id:
            lang = partner_obj.browse(cr, uid, partner_id).lang
        context_partner = {'lang': lang, 'partner_id': partner_id}

        if not product:
            return {'value': {'th_weight': 0,
                'product_uos_qty': qty}, 'domain': {'product_uom': [],
                   'product_uos': []}}
        if not date_order:
            date_order = time.strftime(DEFAULT_SERVER_DATE_FORMAT)

        result = {}
        warning_msgs = ''
        product_obj = product_obj.browse(cr, uid, product, context=context_partner) # product_obj definition
        
        result['discount'] = product_obj.x_discount # modification

        uom2 = False
        if uom:
            uom2 = product_uom_obj.browse(cr, uid, uom)
            if product_obj.uom_id.category_id.id != uom2.category_id.id:
                uom = False
        if uos:
            if product_obj.uos_id:
                uos2 = product_uom_obj.browse(cr, uid, uos)
                if product_obj.uos_id.category_id.id != uos2.category_id.id:
                    uos = False
            else:
                uos = False
        fpos = fiscal_position and self.pool.get('account.fiscal.position').browse(cr, uid, fiscal_position) or False
        if update_tax: #The quantity only have changed
            result['tax_id'] = self.pool.get('account.fiscal.position').map_tax(cr, uid, fpos, product_obj.taxes_id)

        if not flag:
            result['name'] = self.pool.get('product.product').name_get(cr, uid, [product_obj.id], context=context_partner)[0][1]
            if product_obj.description_sale:
                result['name'] += '\n'+product_obj.description_sale
        domain = {}
        if (not uom) and (not uos):
            result['product_uom'] = product_obj.uom_id.id
            if product_obj.uos_id:
                result['product_uos'] = product_obj.uos_id.id
                result['product_uos_qty'] = qty * product_obj.uos_coeff
                uos_category_id = product_obj.uos_id.category_id.id
            else:
                result['product_uos'] = False
                result['product_uos_qty'] = qty
                uos_category_id = False
            result['th_weight'] = qty * product_obj.weight
            domain = {'product_uom':
                        [('category_id', '=', product_obj.uom_id.category_id.id)],
                        'product_uos':
                        [('category_id', '=', uos_category_id)]}
        elif uos and not uom: # only happens if uom is False
            result['product_uom'] = product_obj.uom_id and product_obj.uom_id.id
            result['product_uom_qty'] = qty_uos / product_obj.uos_coeff
            result['th_weight'] = result['product_uom_qty'] * product_obj.weight
        elif uom: # whether uos is set or not
            default_uom = product_obj.uom_id and product_obj.uom_id.id
            q = product_uom_obj._compute_qty(cr, uid, uom, qty, default_uom)
            if product_obj.uos_id:
                result['product_uos'] = product_obj.uos_id.id
                result['product_uos_qty'] = qty * product_obj.uos_coeff
            else:
                result['product_uos'] = False
                result['product_uos_qty'] = qty
            result['th_weight'] = q * product_obj.weight        # Round the quantity up

        if not uom2:
            uom2 = product_obj.uom_id
        # get unit price

        if not pricelist:
            warn_msg = _('You have to select a pricelist or a customer in the sales form !\n'
                    'Please set one before choosing a product.')
            warning_msgs += _("No Pricelist ! : ") + warn_msg +"\n\n"
        else:
            price = self.pool.get('product.pricelist').price_get(cr, uid, [pricelist],
                    product, qty or 1.0, partner_id, {
                        'uom': uom or result.get('product_uom'),
                        'date': date_order,
                        })[pricelist]
            if price is False:
                warn_msg = _("Cannot find a pricelist line matching this product and quantity.\n"
                        "You have to change either the product, the quantity or the pricelist.")

                warning_msgs += _("No valid pricelist line found ! :") + warn_msg +"\n\n"
            else:
                result.update({'price_unit': price})
        if warning_msgs:
            warning = {
                       'title': _('Configuration Error!'),
                       'message' : warning_msgs
                    }
        return {'value': result, 'domain': domain, 'warning': warning}

Hope this helps.