There is also an alternative approach, one that better protects the person who has found a solution. There could be a (reasonable) concern by the tasker that since you (presumably) are a new client (employer) on Microlancer, that after they do the work to provide a valid solution, and are the first, that no payment by you would actually occur.

So when creating the task listing, instead of choosing "no escrow", you could keep the default escrow setting but then (clearly) state that the solution should be prepared by the tasker (worker) before they submit an offer.

When you accept a tasker's offer, you would then pay the LN invoice for the Bounty amount (which gets held in escrow by Microlancer). After that, the tasker would reveal their solution to you (e.g., in the notes, or as an attachment). If that solution is acceptable to you, then you release the escrow to the tasker.

So that's another approach to offering a bounty on Microlancer.