Where and when should I report a $500 State income tax expense for Tax year 2015, which i paid in Feb 2019 (with no reason code).

 

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Next year report it on your 2019 federal return as an Itemized Deduction.
Federal Taxes (or Personal for desktop H&B version)
Deductions and Credits
Then scroll way down to Estimates and Other Taxes Paid
Other Income Taxes - Click the Start or Update button

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Thanks!
Anonymous
Not applicable

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realize it will only benefit you for 2019 if 1) you itemize and 2) if your schedule A taxes before this payment are less than $9,500.  between 9500 and 10,000 partial benefit, over 10,000 no benefit whatsoever.