Von der Perfomance her etwas schwierig, aber machbar.

select a.key, sum(a.Wert) from myfile a

where not exists (

select * from
(
select x.Key from myfile x
inner join myfile y
on x.key=y.key and abs(x.wert) = abs(y.wert)
) z where a.key=z.key
)

group by key