select SUM(bytes/1024/1024) from dba_data_files where TABLESPACE_NAME='UNDOTBS1'

SUM(BYTES/1024/1024)
--------------------
            7000

select SUM(BYTES/1024/1024) from DBA_UNDO_EXTENTS where STATUS LIKE 'ACTIVE';

SUM(BYTES/1024/1024)
--------------------
               8

select SUM(bytes/1024/1024) from dba_data_files where TABLESPACE_NAME='UNDOTBS1'
minus
select SUM(BYTES/1024/1024) from DBA_UNDO_EXTENTS where STATUS='ACTIVE';

SELECT
   (SELECT SUM(bytes/1024/1024) as sum_a
    from dba_data_files where TABLESPACE_NAME='UNDOTBS1')
 - (select SUM(BYTES/1024/1024) as sum_b
    from DBA_UNDO_EXTENTS where STATUS LIKE 'ACTIVE') as Difference
FROM
    dual;

SELECT sum_a-sum_b as result
FROM
    (SELECT SUM(bytes/1024/1024) as sum_a from dba_data_files where TABLESPACE_NAME='UNDOTBS1') as a,
    (select SUM(BYTES/1024/1024) as sum_b from DBA_UNDO_EXTENTS where STATUS LIKE 'ACTIVE') as b

SELECT ...
minus
SELECT...

select (SUM(u.bytes) - SUM(a.BYTES)) /1024/1024
from dba_data_files u
   cross join DBA_UNDO_EXTENTS a
where u.TABLESPACE_NAME='UNDOTBS1'
   and a.STATUS='ACTIVE';