You did not mention the state in which this is taking place, and of course that matters greatly because property tax laws are state laws. Note that there is no conflict of interest for the county attorney here because a conflict of interest would be something that would impair his ability to zealously represent a client. He does not have that problem here. Whether there might be some other kind of ethical problem with his conduct would depend on the laws and rules of conduct used in your state.

The basic issue is whether the county attorney is correct that your organization does not qualify for exemption from the property tax. An attorney familiar with property tax law in your state should be able to answer that. If there is a case to be made that you qualify for the exemption there should be a process for you to appeal the county's determination, but if you missed any deadlines for doing that your organization might be stuck. Of course, all this would require your organization to be able to pay for the legal help needed to mount that sort of challenge.