I have seen an answer which "has been awarded bounties worth 25 reputation by Community".
But according to the Help center, the minimum reputation is 50.
Then, how is it possible?
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Where a bounty is not awarded by its creator it is awarded automatically. In this case only half the amount is awarded.
This is covered withing the bounty FAQ:
What is automatic awarding?
Approximately 24 hours after the end of the bounty period, if the bounty starter has not manually awarded the bounty, the bounty may be awarded automatically.
If the bounty starter accepted an answer during the bounty period, that answer is awarded the bounty. Answers accepted before the bounty period are not eligible to be awarded the bounty automatically.
Otherwise, if there are eligible answers, the highest scoring is awarded half the bounty amount. The criteria for an answer to be eligible are:
- The answer must have been given after the bounty was started
- The answer must have a score of at least +2
- The answer must not have been written by the bounty starter
If two or more eligible answers have the same score (if their scores are tied), the oldest answer is awarded the bounty.
If neither of these conditions apply, the bounty is not awarded to any answer, and is not refunded to the bounty starter.
If the person who started the bounty doesn't award it themselves, under some conditions the system will award it - when that happens, half the reputation is awarded.
So, this happened with a 50 rep bounty.
See How does the bounty system work? for details.