This one just appeared: https://unix.stackexchange.com/questions/37780/paid-help-for-recovering-deleted-files-after-server-reinstall , where the asker proposes to pay for help
This has been already discussed in the Meta Stack Overflow and seems that the consensus is that this is not allowed, e.g.
Offering actual money as a bounty? Pay money to SO for quick support
How to behave in this situation?