From "What to do when OP offers payment for solution?" on stackoverflow meta --
Whatever you want to do offsite is your own business, but the community frowns on monetary bounties for questions.
If you see language like this in a question, feel free to edit it out to focus on the meat of the question. If it's in a comment, flag the comment as noise and we'll get rid of it.
I wouldn't recommend downvoting or closing a question just because this had it in it. Again, if editing that out leaves behind a good question, do so and judge the question on the result.
See also What happens if a user offers money for an answer? with much the same answer,
and How should one handle posts offering money? on Mathematics.SE Meta (similar answer).
There doesn't seem to be a policy against offering a reward, but it certainly runs counter to the spirit of SE, and we don't want to encourage people to start offering off-site rewards (that cannot be verified or enforced) for on-site activity, for hopefully obvious reasons. As some of the other linked answers note, repeated attempts to do this might draw unwanted attention from Moderators, but for a single occurrence, it's probably best to edit the offer out and ask the poster not to do that.
In this specific case, where apparently solving the puzzle means unlocking the reward, it's hard to see how one might disentangle the puzzle from the reward offer. The puzzle will presumably be as interesting to solve whether the reward is actually present (or still up for grabs) or not when a solver solves it, so the puzzle in its own right is fine, but there doesn't seem to be a way to edit the reward out of the puzzle if it's intrinsically part of it.
Anyone have any suggestions?