I would put it on the returns and allowances line. Line 2 on Sch C
But try and get the corrected 1099 so IRS would have a copy of the corrected amount. That would be the best solution.
Also, as @SweetieJean said, check your credit reports, etc. for possible hacking.
Definitely let the issuer of the 1099-K that it is wrong. Perhaps they have a systemic problem and other 1099-Ks issued as wrong too.
**Disclaimer: Effort has been made to offer correct information; but due to the discussion forum limitations, the poster disclaims any legal responsibility for the accuracy of the poster's response**
