BountySource does this, with an approval step to confirm that the PR met the issue requirements. https://www.bountysource.com/ https://www.bountysource.com/ I’

BountySource does this, with an approval step to confirm that the PR met the issue requirements. https://www.bountysource.com/ https://www.bountysource.com/ I’ve used it twice and it worked as expected, though the audience of people doing work is fairly small.