Currently, Trulia does not have the tools to correct individual sales transactions. That said, this is a known issue and we are working towards creating a better experience for our consumers. What we can do to as a workaround is to remove the entire Price History section. Although we do not have the ability to fix or remove individual dates, we do have the ability to remove the entire section. This will remove every price history from display on your property page including the incorrect transaction. Although I understand this is not the most ideal solution, this is something we can do for you to help resolve this as best as we can. If you would like to proceed with removing this section, please let me know.
For future reference, you can feel free to contact us about this type of inquiry through our contact form here:
Thank you for using Trulia!
Consumer Care Advocate