Per IRS Publication 17 Your Federal Income Tax (For Individuals), page 189:
If you figured your casualty or theft loss using the amount of your expected reimbursement, you may have to adjust your tax return for the tax year in which you receive your actual reimbursement. This section explains the adjustment you may have to make.
Actual reimbursement less than expected. If you later receive less reimbursement than you expected, include that difference as a loss with your other losses (if any) on your return for the year in which you can reasonably expect no more reimbursement.
Example. Your personal car had an FMV of $2,000 when it was destroyed in a collision with another car in 2017. The accident was due to the negligence of the other driver. At the end of 2017, there was a reasonable prospect that the owner of the other car would reimburse you in full. You didn't have a deductible loss in 2017.
In January 2018, the court awarded you a judgment of $2,000. However, in July it became apparent that you will be unable to collect any amount from the other driver. You can deduct the loss in 2018 (to the extent it doesn’t exceed your 2018 personal casualty gains) subject to the deduction limits discussed later.