On this question the OP has given a bounty worth 550 rep but at the moment I have 844 rep and I can't give any more than 500 rep on a bounty.
Can you please tell us if this is a bug or not?
This is not a bug,
When that bounty was started, the system added a
According to the Blog (Improvements to Bounty System), this bonus was removed in June 2010:
The old reputation system included an incentive 50 rep, while the new system does not.
The reason you can't give more than 500 rep is to prevent bounties from becoming a trivial method to gain significant moderation abilities. They are there to incentivize giving great answers, but are not meant to vault one up too far along the reputation ladder.