thanks for you reply Eric. But I'm a bit lost in figuring out how to accomplish your suggestion. If you have some spare time, can you provide me some specific details?

This is the _make_invoice() method in sale.py:

    def _make_invoice(self, cr, uid, order, lines, context=None):
        inv_obj = self.pool.get('account.invoice')
        obj_invoice_line = self.pool.get('account.invoice.line')
        if context is None:
            context = {}
        invoiced_sale_line_ids = self.pool.get('sale.order.line').search(cr, uid, [('order_id', '=', order.id), ('invoiced', '=', True)], context=context)
        from_line_invoice_ids = []
        for invoiced_sale_line_id in self.pool.get('sale.order.line').browse(cr, uid, invoiced_sale_line_ids, context=context):
            for invoice_line_id in invoiced_sale_line_id.invoice_lines:
                if invoice_li d.invoice_id.id not in from_line_invoice_ids:
                    from_line_invoice_ids.append(invoice_line_id.invoice_id.id)
        for preinv in order.invoice_ids:
            if preinv.state not in ('cancel',) and preinv.id not in from_line_invoice_ids:
                for preline in preinv.invoice_line:
                    inv_line_id = obj_invoice_line.copy(cr, uid, preline.id, {'invoice_id': False, 'price_unit': -preline.price_unit})
                    lines.append(inv_line_id)
        inv = self._prepare_invoice(cr, uid, order, lines, context=context)
        inv_id = inv_obj.create(cr, uid, inv, context=context)
        data = inv_obj.onchange_payment_term_date_invoice(cr, uid, [inv_id], inv['payment_term'], time.strftime(DEFAULT_SERVER_DATE_FORMAT))
        if data.get('value', False):
            inv_obj.write(cr, uid, [inv_id], data['value'], context=context)
        inv_obj.button_compute(cr, uid, [inv_id])
        return inv_id