To really make it fair, you'd need some way to collect the bid(s) in advance
and put them in escrow
before the developer started work.
Once completed, you'd also need some mechanism (possibly a vote or impartial reviewer?) to determine if the fix, as submitted, met the criteria specified. And then you'd need a mechanism for releasing the collected bids to the programmer. Sort of like how EBay operates.
Without some neutral and trusted party in the middle holding the money - and ultimately signing off on the actual delivery - it's much too easy for either side to default on their side of the deal.
But acting as an escrow agent also incurs certain legal responsibilities. And expenses. It's not the sort of thing that could be handled with nothing more than a handshake since there are state and federal statutes governing that sort of thing. And it certainly couldn't be done for free since expenses will be incurred, both for administrating it, and to comply with the law...
This is the problem I see with many of these really good ideas. They try to handle an essentially formal business arrangement on an informal basis. Most times it doesn't work long-term. It's only a matter of time before somebody breaks the covenant and the whole thing falls apart. Or they get into some sort of legal or regulatory trouble. (Look how the FTC and SEC are now paying serious attention to crowdsourcing in the wake of Amanda Palmer's spectacular $1.2 million Kickstarter success story.)
But hope springs eternal.
I wish them the best of luck!