돌려본 것은 아니지만 아래와 같이 하시면 될거예요.

select

a.no, a.name , a.금액, (a.금액 - b.금액) as a_b

from

table_a a

, table_b b

where

a.no = b.no

요렇게 하시면 됩니다.

drop table a;
create table a
(a1 char(1),
 a2 varchar2(10),
 a3 number);

insert into a values ('1','사원A',10000);
insert into a values ('2','사원A',20000);
insert into a values ('3','사원B',30000);

drop table b;
create table b
(b1 char(1),
 b2 varchar2(10),
 b3 number);

insert into b values ('1','사원A',15000);
insert into b values ('3','사원B',30000);

commit;

select a1,a2,a3,
       case sign((a3 + a4) - b3)
       when 1 then b3-a4
       else a3 end "-하고자 하는 값"
           
from
(
select a1,a2,a3,a4,b3
from
(
select a1,
       a2,
       a3,
       nvl(sum(a3) over (partition by a2 order by a1 rows between unbounded preceding and 1 preceding),0) a4
from a
) a,
b
where  a.a2 = b.b2(+)
order by a1
)