how about this one. try this one. and compare your table with this one.
CREATE TABLE [dbo].[service](
[jobno] [varchar](5) NULL,
[amount] [smallint] NULL
) ON [PRIMARY]
GO
Insert into service(jobno,amount)
Select 'j211',2 UNION ALL
Select 'j211',2 UNION ALL
Select 'j211',-4
select sum(Amount)
from service
where jobno = 'j211'
group by jobno