I recently flagged a question (this one) as really bad (because it was). However, I also left a comment for the original poster letting them know that it was really vague and not a good fit. The poster almost immediately edited the post and improved it so that my flag is no longer necessary. The mods get a lot of things they have to deal with, so I wanted to rescind my flag now that the issue had been dealt with, but I don't think there's a way to do so. It would be cool if I could go to the list of flags I've set and select "Clear" or "Remove" or something. And maybe leave a comment about why so that any mod who was reviewing the flag could see I wasn't just being a tool flagging and un-flagging things.
There's no way to cancel a flag at the moment, the current best way to do this would be flag the post with an "other" flag and explain the situation.
Remember if the post obviously alright your flag will probably just be marked helpful/declined very quickly, which isn't much work. And there's not much reason to be concerned about declined flags (unless the majority of your flags are declined, but that's quite a rare case).