class SaleAdvancePaymentInv(models.TransientModel):

    _inherit = "sale.advance.payment.inv"

    @api.multi

    def _create_invoice(self, order, so_line, amount):

        inv_obj = super(SaleAdvancePaymentInv, self)._create_invoice(order, so_line, amount)

        inv_obj.write({'service_id':order.service_id.id})

This is my code.service id does from from sale order to invoice through invoiceable lines.

But when I use downpayment the service id is passed to invoice.

What is the reason behind this.?
How to pass the values through the invoiceable line?