You are correct that you do not want to change your W-2.
The $2,900 appears in box 10 on your W-2.
In the W-2 interview, you will be asked if you had an FSA (yes) and if there was anything left in the FSA (answer, zero or 0).
In the Child and Dependent Care interview, enter the $850 as paid to the caregiver. You have nothing carried over from last year and are carrying over nothing to next year. The net result is that $2,050 is unaccounted for or just disappears. This $2,050 gets taxed, and is added back to Income on line 1 on your 1040. You have no Child Tax Credit because the amount given to the care giver was either excluded from income or was forfeited.
This should give you the result you want without having to change the W-2.
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