Having obtained permission to cut down a neighbor's trees encroaching our property, the neighbor has rescinded permission.

Well, if there was only oral permission, it will probably be a he said/she said kind of affair. The real question, as far s any downside for you, is whether you trimmed or actually cut down the trees, and how many trees and what kind they are. If you only trimmed trees that encroached and littered on your property, you rally have no downside. If you actually went onto their property and cut down the trees, it will probably be worth hiring a local lawyer for an hour or two to assess your situation

I assume the permission was oral only because you mentioned no writing. However, you acted in reliance upon their permission and so for them to "change their mind" is a breach of their agreement. I do not believe there is a court or jury who will not believe you had some sort of permission prior to cutting the trees. Some states actually have a cause of action for "timber trespass", and I am not licensed in your state, however consent is always a defense in any trespass case. Hire an attorney and defend the case. File a Motion to Dismiss and air that out prior to filing any answer. This answer is provided for informational purposes only. Actual legal advice can only be provided in an office consultation by an attorney licensed in your jurisdiction, with experience in the area of law in which your concern lies.