Out put 

45

output should be '00'

select                    LPAD (
                      CASE
                         WHEN ACC_NO = '71'
                         THEN
                            round(TO_NUMBER (
                               replace (replace (amount,'-',''),'.','') ),0) end ,8)     ||
                      rpad(replace(substr(amount,instr(amount,'.')+1),'-',''),2,'0')
from dual;

SQL> WITH DATA AS(
  2  SELECT '-123.45' num FROM dual UNION ALL
  3  SELECT '123.45' num FROM dual UNION ALL
  4  SELECT '-123' num FROM dual UNION ALL
  5  SELECT '123' num FROM dual UNION ALL
  6  SELECT '-0.45' num FROM dual
  7  )
  8  SELECT num,
  9    CASE
 10      WHEN TRUNC(num) = num
 11      THEN '00'
 12    WHEN num - TRUNC(num) <> 0
 13    THEN substr(num, instr(num, '.', 1)+1)
 14    END num_case
 15  FROM DATA
 16  /

NUM     NUM_CAS
------- -------
-123.45 45
123.45  45
-123    00
123     00
-0.45   45

SQL>