You don't sponsor people or projects to complete specific issues or build specific features in the first place. Sponsorship is a reward and token of appreciation for doing good work.

Then you offer to pay the maintainer their consulting rate to do it if they are willing.

You hit up the maintainer and negotiate a deal for that?

If all you’ve got is relative pocket change they probably aren’t going to agree but if you put real money behind it and it doesn’t go against their vision of the project then most people would be willing to accept actual contracting work to expand their project.

You actually hire a developer to work on that issue and not something else.