How does it work?

User offers some money to developers to fix a specific issue.
User doesn't pay immediately, just expresses his intention to pay after the issue is fixed. User can choose any amount.
Bid accumulation
Other people with the same need also bid for the same fix.
Developer decision and fix
Developer prioritizes possible fixes taking into account all kinds of motivation including the bids. If an issue rises high enough in priorities of the developer it eventually gets fixed.
Delivery and review
A new version of the project is made available free for everybody as usually. Developer claims the fix delivered.

User reviews the new version with regard to the fix and decides how well did it meet his expectations. After the review the user has two options.
In case the user likes the fix he pays the money offered in the first step. The payment is processed by PayPal and received directly by the project developer.

The user gains the fix and its advantages, good feeling he supported the project and motivate developer to keep up the good work.
As this is an experimental service the users can opt-out and cancel their bids. They are only required to leave feedback.