My employer claims my DCFSA contribution was higher than the actual allowance in the account. Have do I file the tax return and how can I recapture the difference?

The value on my W-2 Box 10 is higher than what was allowed to me through the FSA/DCFSA account (Amben or https://americanbenefitsgroup.wealthcareportal.com/).
The employer verified the numbers and informed me Amben is going to correct the balance. However, I can no longer use the balance from the previous year and on top of that this account/employer is no longer active.
How can I recapture this money?
I specified in TT I did not use all my DCFSA funds but my "federal tax due" was not adjusted for the difference or any difference at all.
Thanks

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