[Can] I voluntarily return the whole , half or at least a token sum to the user who offered the bounty[?]
As far as I know, you cannot give your reputation directly to someone else. It would game the system if such a provision exists. People, when they feel extremely generous to a user, tends to serially upvote that user's posts. This is also considered an illegitimate act in the network which shouldn't be considered either.
Besides, the act of having that bounty ended up in your account doesn't seem to be a technical loophole but a manual act by OP himself.
Since your answer had three upvotes during the time bounty was active, your answer was already eligible for automatic (half) bounty by Community ♦ user and possibly would've been awarded if OP couldn't have offered it manually until the grace period ended.
What's more? I believe OP did the right thing. Bounties should be awarded to answers which either solved the issue or to those whom the community has upvoted well. It is not mandatory though.
Beska has written here:
If the person doesn't want to accept an answer, because their problem isn't yet solved, then award the bounty to the highest rated answer as normal, but don't auto-accept an answer. Then the questioner could hope for more answers, put up more information, perhaps a second higher bounty, etc.
There are conflicting answers as well. See them here: Why do I always have to accept an answer at the end of the bounty period?.
Now, coming to the point. You can choose to offer a chunk of your reputation points to a user, yet not violating the policies of network, by choosing to offer a bounty to an exemplary answer of that user (a direct transfer) or by offering a bounty on that user's question. The latter wouldn't transfer the points but would help that user greatly. To avoid abuse of system, if you choose the former technique, you would have to wait for 24 hours to award the bounty.
OP in this case doesn't seem to have any answer posted on the site, let alone one to be considered exemplary, so you can't go for the former technique. Only option left would be to help him out by using the latter technique.
Alternately would a better solution be for me to delete my answer?
As mentioned by Lucky, you shouldn't and need not to delete the post, although I'm not sure whether you can simply delete an answer or would have to vote and wait for it to reviewed, in case it is an upvoted answer.
I believe you shouldn't be having that honesty part troubling you anymore, now that it is clear in my reasoning noted above. I would like you to note that your answer falls in the category of the answers termed as partial answer. Help center states here:
(Emphasis mine)
Help us find a solution by researching the problem, then contribute the results of your research and anything additional you’ve tried as a partial answer. That way, even if we can’t figure it out, the next person has more to go on.
This is what you simply did, and it's has been considered worthy, hence, you've got 140 points only from a partial answer.
I recommend you put a note in the beginning or the end of the post, stating that "this is a partial answer" so that visitors in the future might not get confused as to why your answer (considering that you do not end-up solving the issue) when not solving the issue completely has been upvoted as well as earned the bounty.
Further readings:
