There's a question offering +100 reputation, but the user doesn't have this amount of reputation

Question

Is this a bug? or is this entirely possible?

Answer 1

The bounty amount is subtracted from the user's rep when the bounty is offered, not when it is rewarded.

In other words: The user did have over 100 reputation when he offered the bounty and now he doesn't anymore because the 100 were already subtracted.