The existing answers by Mari-Lou and NVZ are entirely correct. The canonical FAQ is on MSE: How does the bounty system work? This answer is correct as of the time of posting, and is specific to the circumstances described.
Bounties are bounties, and it's not possible to alter them once they have been set. Moderators do have some abilities, but they don't cover amending bounties.
In some circumstances, it's possible to cancel bounties, but the only circumstances where a bounty might be cancelled are to allow an egregiously off-topic question to be closed, or where the bounty notice is offensive, or some similar exceptional case. Your question is on-topic and there is no reason to cancel the bounty.
A bounty can be manually awarded by the donor after its period has ended, to the answer the donor feels deserves it. If that doesn't happen within a reasonable time, then the system makes the decision: half of the amount is awarded automatically to the highest-voted answer written by someone other than the donor, scoring at least +2, which was posted after the bounty started. The bounty donor cannot receive the bounty, even if his answer is otherwise eligible.
In this case, there is an answer which is eligible for that half-bounty award, so it's not appropriate for moderators to take action which could prevent that happening. That is, it would not be appropriate to cancel the bounty only to find that you didn't reinstate it. Indeed, if mods were to cancel the bounty and you did reinstate it, that answer would not be eligible for an automatic award based on the increased amount, because it was posted before the new bounty.
Currently, one answer will get half the bounty if you don't award all of it. There is nothing stopping you starting another bounty; and, if you want to award the new bounty to a specific answer, there is a bounty notice which explicitly indicates that ("One answer is exemplary..." or similar wording).
So: if you think the existing eligible answer is worth more than +100, award the existing bounty explicitly and then start another bounty with the intention of awarding that as well. As NVZ has written, the new bounty will need to be double the previous one.
If you don't think the existing eligible answer is worth even +50 of the 100 you have sacrificed, I'm afraid you're out of luck. That's the gamble with bounties: a bounty effectively buys an advertisement, and someone who answers the ad well is eligible for a reward which comes out of that fee.
Note that if you wait for the current bounty to end and then start a new bounty, only answers posted after the new bounty starts are eligible for the automatic half-award. So if you feel that it may be possible to get even better answers with a new bounty, you can encourage them. It's not necessary to post a new question; but if you do feel another question would help, it needs to be sufficiently different that it's not a duplicate.