Michael Davis

Link Away makes it easy to replace a post’s permalink with any URL you choose on a post by post basis.
Link Away adds a field to the post admin page for entering the new destination address.
If you are sharing an article found on another website, all you have to do is add it’s URL. When the user clicks any link that would normally take them to the single post page for that post, they will bypass the post on your site and go directly to the article location.
The value of this plugin compared to others that might be similar, is that it is clean and lightweight. It’s so simple, in fact, that the actual functionalitly is only about a half dozen lines of code.
Because it filters the WP function that retrieves the permalink and replaces it with the given URL as it is returned, with a very high priority, it should have no problem working across all themes and regardless of other plugins.

Link Away

Link Away makes it easy to replace a post's permalink with any URL you choose on a post by post basis.

Burhan Nasir

This plugin allows you to make a WordPress page (or post or custom post type) link to a URL of your choosing, instead of its WordPress URL. It also redirects people who go to the old (or “normal”) URL to the new one you’ve chosen.

Linkoryx – Custom Post & Page Links

Make any page, post, or custom post type link to a URL of your choosing instead of its WordPress URL.

Link Away vs Linkoryx – Custom Post & Page Links