accrue state income tax on schedule A ?

New client , owed $ 100k of state income taxes for 2017 - paid in April with extensions. Finalizing the returns looking for any way out of losing this deduction. Clearly, is cash basis taxpayer ( and not eligible for automatic change to accrual since no schedule C ). Trying to see a way for an automatic change in accounting method to accrual ( for at least this item) . I think I am basically wasting my time, but $ 40 k of federal tax benefit -seems ridiculous to lose....

"looking for any way out of losing this deduction"
Nope, it's gone.

It's gone, and thanks to the tax act, it is also capped at $10k for 2018. Bye-bye the other $30k plus any other 2018 amounts paid to state.

Just to confirm this activity related to his personal return and not a business return .

Was there a unique tax event that occurred in 2017 that caused him to owe the 100 K or is this normal activity. Also if a unique event was your client fully aware of it before December 31,2017 ?