You can enter the negative number as an "Other Income" entry.
Enter (or leave the entry for) the W-2
Click on Federal,,,Wages & Income,,,,scroll
down to the last section "Less Common Income"
Scroll down to the last choice "Miscellaneous
Income, 1099-A 1099-C" and Start
Scroll down to the last choice "Other reportable
income" and Start
For the Description enter "TAX TREATY"
Enter the amount of the income that you are attributing to
the treaty AS A NEGATIVE NUMBER
Print the 1040X
Mail with form 8833. TurboTax does not supply Form 8833, you will need to print one and attach it to your 1040X.
https://www.irs.gov/pub/irs-pdf/f8833.pdf
https://www.irs.gov/forms-pubs/about-form-8833
https://ttlc.intuit.com/replies/6700955
https://ttlc.intuit.com/replies/3288565