After you file

I don't have a Mac. I can show you how it works in Windows. If this doesn't solve the problem I'll try to get someone who uses a Mac to help you.

On the investment summary screen do you not have an Edit button to the right of the summary line for the brokerage? See the first screen shot below. It's all the way to the right. You might have to scroll to the right or maximize your TurboTax window to see it.

When you click Edit for the brokerage, you get a screen that lists the individual sales that you entered. See the second screen shot below. To delete a sale, click the trash can icon to the right of the sale you want to delete. Again, it's all the way to the right.