To expand on this: you don't need to manage them. You don't need to track their progress. You don't need a special team for them, or a 'head of open-source'.
You pay them a salary in the same way that you would pay a salary to eg. someone who you don't really have any work for, but don't want to see leaving for a competitor either: you add them to payroll and just let them do their thing.
They're already a maintainer so they know how to manage the project. There are no further expenses or organizational overhead for you.