Adverse possession

I live in Washington state. I am buying a home. There is a fence encroaching on the property. To the best of my knowledge it has been there for at least 10 years. There has been no legal action taken be the encroaching party to take adverse possession. Would a contract between the encroaching party and I that stated that I would allow the fence to stay in place if they would agree not to seek to gain the property by adverse possession be an appropriate solution? If so are there any forms on this site that I could use.

I've seen the encroaching party provide a quit claim of any interest in the encroached upon property, which is then recorded.