You should have gotten 2 emails when the guest made the original reservation: the booking request notification and the booking confirmation notification. The booking request notification has more guest info in it than the confirmation, and shows exactly what the guest ultimately paid when booking was accepted, including taxes, Airbnb guest fee, cleaning fee, etc. Hopefully you have retained those emails.
If not, this is a long shot, but most (good) software development shops have an event log they can consult to see all of the actions that have occurred on your account and/or listings. It takes some diligent programming, but there might be a “Host changed price from x to y”, or “Host accepted reservation at nightly rate of x” type of message in the logs for your account. You could call CS and ask them if they have access to such a log.