I posted a question on math.SE a few weeks back, which received a fair bit of attention and some promising leads in the comments but no clear answers. I posted a +100 bounty on the question, and during the bounty period an answer was posted which provided some mildly interesting discussion of why $6!\cdot 7!=10!$ but did not provide anything resembling a bijection, which was the explicit aim of the question and associated bounty. The bounty has now expired with no additional answers.
Because the question has been fairly popular, the answer has been upvoted by 4 of the 684 viewers of the question (and downvoted by one), even though it doesn't seem like the community's overall opinion is particularly favorable - the question itself was upvoted 16 times over the same period since the answer was posted. As such, it will receive a +100 bounty if I manually award it, and a +50 bounty if I let the bounty expire, per my understanding of the bounty system.
I am unsure of the proper response here. On the one hand, I appreciate the answer, and found it somewhat interesting - I would rather have read it than not. (I haven't voted on it either way.) I also don't want to let the 100 reputation "go to waste", as it were. This would suggest directly awarding the bounty.
On the other hand, the answer does not even attempt to provide the bijection my question was looking for, and it seems like it would be bad for the site to set the expectation that posting incomplete answers on popular-but-hard bountied questions is a way to earn easy reputation. It feels sort of unfair to me to award the same bounty to an incomplete answer that I would have liked to award to a full response to the question. This would suggest not awarding the bounty, but as the voting patterns make this impossible, letting it expire for half-credit as a fallback plan.
Right now, I am thinking of letting the bounty expire and giving +50 reputation to the existing answer, and editing my question or adding a comment to make clear that I will create and award a new +100 bounty to any future answer that resolves this question. Is this a reasonable choice to make? Are there factors I'm neglecting, or any other thoughts on what the right choice is in this situation? I'm relatively new to the site and don't want to violate any math.SE norms or slight the user who posted an answer, if possible.