As I see it, there are really two issues here.
- Should people be allowed to offer monetary or other non-Stack Exchanged based rewards, for answers to their questions?
- Should people be allowed to advertise the offers here on Ask Ubuntu?
It seems most people would say "no" to the second question, and arguably this would even qualify as spam under the current definition.
But if people want to use some separate web-based (or other type) platform to offer to give people money, or property, or advertising space, or sing them a love song, or whatever else, in exchange for answering their question here? Sure, why not. If Ask Ubuntu is not being used to make the offer, we really have no legitimate reason to object (nor any legitimate reason to encourage the practice).
So, I think:
People should not offer those incentives here and it's perfectly good to edit them out of questions or flag comments that advertise them as not constructive / off-topic.
Even posting a link on main to a page about the award being offered should not be allowed.
I'm not sure what the policy should be in chat, but I'd lean toward prohibiting its use for that purpose also.
But if people want to offer them elsewhere--for example, share a link off-site, then that's fine. In this situation, we are not the platform on which the offer is being made, nor the platform on which it is redeemed.
If there is a dispute, it doesn't fall to us. If people try to use us to resolve disputes, we can politely but firmly tell them that neither Stack Exchange nor the Ask Ubuntu community is a party to their arrangement, and that we will not allow ourselves to be pulled into it.