Thanks, DianneW777.
Apparently Schedule A, line 5a instructions are simplistic and over-riden by Pub 525.
The latest Pub 525 (2024) example 28 covers this case:
Example 28. You paid 2023 estimated state income tax of $4,000 in four equal payments. You made your fourth payment in January 2024. You had no state income tax withheld during 2023. In 2024, you received a $400 tax refund based on your 2023 state income tax return. You claimed itemized deductions each year on Schedule A (Form 1040). You must allocate the $400 refund between 2023 and 2024, the years in which you paid the tax on which the refund is based. You paid 75% ($3,000 ÷ $4,000) of the estimated tax in 2023, so 75% of the $400 refund, or $300, is for amounts you paid in 2023 and is a recovery item. If all of the $300 is a taxable recovery item, you'll include $300 on Schedule 1 (Form 1040), line 1, for 2024, and attach a copy of your calculation showing why that amount is less than the amount shown on the Form 1099-G you received from the state. The balance ($100) of the $400 refund is for your January 2024 estimated tax payment. When you figure your deduction for state and local income taxes paid during 2024, you'll reduce the $1,000 paid in January by $100. Your deduction for state and local income taxes paid during 2024 will include the January net amount of $900 ($1,000 − $100), plus any estimated state income taxes paid in 2024 for 2024, and any state income tax withheld during 2024.
TT is correct and Schedule A instructions are incomplete at best.
