GitHub OSS projects usually manage this via GitHub Sponsors.
See this example.
In this case Alex creates an issue that he is willing to pay for and a developer creates the PR assigning himself to work on it.
That said, the maintainer/petitioner could end not paying or, like Sergi said, there is no guarantee of merging the developers work into the main branch of the project.
A common case that could happen would be if the petitioner requests a new feature while being on the LTS 6.0 but new features are only implemented for the (current) 6.7 branch (6.8 to be).
I think maybe Tryton could provide a bounty wall on the forum once/if the discussion on non-official modules settles. The petitioner would create the issue and the bounty post.