I'm bringing up this question again, because the previous one is four years old and the accepted answer is, well, wrong. There seems to be some disconnect between the people who want this feature and the people who don't that I don't entirely understand. The text of the relevant bounty reason is:
Reward existing answer
One or more of the answers is exemplary and worthy of an additional bounty.
The argument is whether such a bounty should be awardable immediately, instead of having to wait 24 hours. I would argue that such a bounty must be awarded immediately; the bounty creation dialog should ask you which answer you're rewarding and award the bounty directly to that answer. Nonsensical arguments against this include:
- The goal of bounties is to draw attention to questions -- True in most cases, but explicitly untrue in this case; the goal is to "reward an existing answer". We don't need new answers, this question already has such a great answer we want to give the person who wrote it a bonus.
- People need time to write a good answer and earn the bounty -- The bounty has already been earned. It was earned before the 24 hours even started, so tricking other users into thinking they have a shot is disingenuous -- the offerer has already mentally awarded it to somebody.
- People will be discouraged if they see a bounty awarded immediately -- Nonsense. If anything people will see "reward existing answer" and think "excellent, any of my answers might get a bounty at any moment if I do a really good job; why even hunt for existing bounties. I should try hard on all my answers".
- There's no way for the system to know that you intend to slap your bounty on an already existing post -- I...what? I don't even know how to respond to this.
- Wouldn't that also allow a fairly unthrottled way of transferring rep? -- That...actually isn't ridiculous, but I'll talk about it below
I think the confusion must come from the word "bounty", which normally has a connotation of being offered before an event. This particular "bounty" is the opposite -- it's a bonus offered after the fact. The fact that it's part of the wider bounty system is coincidental, because there was already a system in place for giving people reputation when they post exceptional answers.
The concern about funnelling reputation to another user is valid, but this method of preventing it is very user-unfriendly. Rewarding an existing answer should take about 6 seconds; users shouldn't need to keep the tab open in the background for an entire day waiting for an arbitrary timer to elapse so they can do what they were going to do anyway. I'm quite sure there are other ways to prevent infinite rep transfer -- only allow a certain number of "reward existing answer" bounties per day, for example, or intervene if the same user is rewarding the same other user over and over, like we do for voting.
TL;DR: Users should not need to wait 24 hours to reward existing answers. At the very least, the bounty should be created as normal and then be immediately rewardable. Ideally, it should just be part of the bounty creation wizard and the bounty should never even be visible on the question.