I need to know whether or not to pay my final est. in 2018 or defer to 2019. If the 2017 state refund is netted against my 2018 est. pmts, then I need to pay the final est. in 2018. If the 2017 refund is treated strictly as income, then I have the max $10,000 for the SALT deduction. And I will be itemizing.
If you itemized your deductions in 2017 then the 2017 state tax refund is added as income on the 2018 Form 1040 Schedule 1 Line 10, it is not entered on Schedule A.
It begs the question: should not the NET SALT over the $10K be the "lost" amount? It's only fair!
if you took a state income deduction rather than a sales tax deduction on your 2017 1040, then the refund might be taxable. it is never netted. why might it be taxable if you took the income tax deduction. if you were in AMT, part of the deduction did not produce a federal tax benefit. OR you might have paid part in 2018 such as an extension or estimate payment, AND there could be other reasons