To answer my own question:
Indeed section 179 deduction is not allowed above business income (it is then carried over to the next year). The solution to my question is to use a $400 partial section 179 deduction AND 100% of the special depreciation allowance.
In this case, the $500 will be deducted this year and $400 will be carried over to the next year (There will be no depreciation).
In my case, it doesn't work as intended anyway because my State does not allow the special depreciation allowance :-(.
