Question : Subtract number and pad as 3 digits in Oracle 9i

Oracle 9i environment.

I need to read the first 3 digits of a field, subtract 400 from it, and ensure the result remains as three digits.
The code I am working with is as follows:
TO_CHAR(TO_NUMBER(SUBSTR(my_field,1,3))-400,'009')

It is returning the correct value, but the next step in the process fails and says it cant read the value. I suspect it is because it is looking for a number, and this has converted it to CHAR.

Normaly the - 400 is not required and the code is simply
SUBSTR(my_field,1,3)  which works perfectly

examples:
523 should return 123
403 should return 003

Any thoughts on how to acheive this and keep it as a number. maybe LENGTH ?

Answer : Subtract number and pad as 3 digits in Oracle 9i

sorry

lpad(TO_NUMBER(SUBSTR(my_field,1,3))-400,3,'0')