This is exactly what https://www.bountysource.com/ does. Any number of people can contribute to the bounty pool. When a developer submits a solution that gets merged into the project and the issue itself is closed, that developer collects the bounty.

For example, here's an issue with 11 backers: https://www.bountysource.com/#issues/27-split-pane-feature

...and here's a bounty over $1100 with 5 backers: https://www.bountysource.com/#issues/315019-add-a-tiling-win...