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Ad when was it paid?  If paid in 2019 for 2018 tax, then it is 2019 income reported next year on year 2019 tax return.

However, this whole thing sounds fishy.    How would they even know what your tax is until you do your tax return?
Did they pay all of your tax, or maybe just a part of the SE tax  (employers share)  as if you were a W-2 employee.?

Sounds like there might be more to this than meets the eye.
**Disclaimer: This post is for discussion purposes only and is NOT tax advice. The author takes no responsibility for the accuracy of any information in this post.**