Actually, the TurboTax program might very well be correct here. In both screen shots - 2016 "as filed" and 2016 "as amended" - you have a relatively small long term capital gain. If your taxable income - including the capital gains - have you in the lowest two tax brackets then long term capital gains are taxed a 0%, meaning that your tax liability was not affected when you entered the sale in 2016 originally and is likewise not being affected when you change the LTCG to a smaller amount with your amendment. I assume that the compensation income created by the sale was on your W-2 and the mistake was that you forgot to add that compensation to your "out of pocket" cost when you originally filed.
Since you are using the desktop version of the program you should be able to confirm this by looking at the tax calculation itself, most likely on the "Qualified Dividends and Capital Gain Tax Worksheet."
Tom Young
