How will the SALT max of 10K be affected by a 2017 state tax refund? Will IRS net the 2017 state refund against my 2018 state tax payments? Or is the 2017 refund income?

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.