Most likely the difference arises because you used a different "fair market value" than the employer used to calculate the compensation. There's absolutely no "Income Tax Return Reporting Requirement" to use the ISO guided interview to report the sale. I'd suggest deleting the trade you've entered and reporting the sale using the "regular" 1099-B interview. Enter the 1099-B exactly as it reads - incorrect basis and all - and then correct the basis to the correct basis - # of shares sold x per share FMV used by the employer - using the mechanism provided by TurboTax to correct basis reported incorrectly on the 1099-B.
