select t.*, sum("In"-"Out") over(order by id) as balance
from tbl t
order by id
Violín: http://sqlfiddle.com/#!15/97dc5/2/0
Considere cambiar los nombres de sus columnas "In" / "Out" para que no necesite ponerlos entre comillas. (Son palabras reservadas)
Si desea solo un cliente (customer_id =2):
select t.*, sum("In"-"Out") over(order by id) as balance
from tbl t
where customer_id = 2
order by id
Si su consulta abarcara varios clientes y quisiera un saldo actual que se REINICIARA con cada cliente, podría usar:
select t.*, sum("In"-"Out") over( partition by customer_id
order by customer_id, id ) as balance_by_cust
from tbl t
order by customer_id, id