A simple way to restrict users to editing only particular posts is to give them the “Author” role, and designate them the given post’s (or page’s or event’s, etc.) “author.” Authorship can be changed at any time, and editors and administrators can still edit those posts.

Post authorship can be changed via the “Quick Edit” menu from the post archive, or on any given post.

To see the Quick Edit menu, hover over the post title and click Quick Edit

You can also change author in post edit: