To clarify, you are now reporting the sale of the RSU stock?
When the stock vested, that would have been included as income on your W-2.
When sold, the basis would be the amount included as income on your W-2 but reported as Capital Gain.
You would adjust the basis if needed.
I do not know what is meant by "tie my RSU income to my W-2 for that employer" means
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