It's not that simple, the id returned for a post should allow to like/unlike the post according to documentation.

Liking works, but unliking does not in cases where the post_id returned has the profile_id prefixed. So you have to remove the profile_id from the post_id to unlike.

The reason why developers have to reverse engineer is that the Facebook platform doesn't and did never work the way it is documented.

Here's a bugreport for that problem which hasn't been solved since last February! (The top comment here with a workaround for that issue was created by myself: https://developers.facebook.com/bugs/539328566085833?browse=...)