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I think it would be great to be able to award a bounty in any amount for an existing answer (to be awarded immediately
Maybe it would also be great to be able to withdraw an arbitrary amount from ATM immediately. Then again, maybe there's a good reason why I can't do that.
Consider the potential for abuse. Should new users get immediate access to moderator tools, if they are buddies with an existing high rep user? Or, should a disgruntled $15$K rep user be able to instantly give $5$ new accounts the ability to close questions? And so on.
why must they come in increments of 50?
Why not? Is there a situation where the bounty of $200$ is too small, $250$ is too big, but $237$ is just right? Allowing arbitrary amounts would require users to key in the value of bounty, which is less convenient and more error-prone.