We had a senior, very capable (i.e. 5x) engineer, "Gust", who has served his notice (4 weeks), and will be leaving. Our company likes to "test" people in their new roles when being promoted for up to 6 months before we officially promote them. Due to incompetence by Gust's former manager, Gust was basically working 2 levels above his pay grade for 18 months, and training new engineers at more senior levels than he's paid (i.e. Gust is a "senior engineer I", and has been training new "senior engineer II/III" positions for over a year).
We severely disciplined the manager that let this drag on for 18 months rather than just 6. The real screw-up (not mine) is that he was training people much more senior than him.
I'm working with limited resources to correct the mess-up someone else created. I tried to ease this over with a promotion to "Senior Engineer II" and a bonus to offset the "missed out pay" for the past 12 months (trial period should never exceed 6 months), but Gust wanted 2-3 promotions outright (senior management declined due to the "optics") and and even greater bonus, i.e. "if I'm training people 2 levels above me, I should be paid at least 3 levels above my current pay grade".
Gust's self-appraisal seems fair, since he's been training people at higher levels. The level he needs to grow into though, requires additional soft skills he does not yet possess. However, since he can train others in much more senior technical roles, he's convinced he already possesses all the soft + technical skills needed.
Gust waited until annual bonuses and stock payouts cleared, and served his notice.
My problem is that, while Gust is providing documentation and training materials for his eventual replacements (had to hire 3x intermediate engineers in his place), a lot of his work depends on changes to open source technologies (numerous open source projects on GitHub). Even though we can gradually train his replacements on these projects (it's just code, after all), a large part of succeeding with these projects involves having someone with the necessary rapport with the maintainers/contributors for these projects. The open source projects have between 5 and 10 maintainers doing most of the work, and Gust is a maintainer on each project.
I don't know what Gust did/said, but most of the developers on these projects (at least those to whom we reached out via e-mail) basically told us off and treated the intermediate engineers with contempt (and in some cases, very foul language). How do we best engage with these open source teams and "pass the torch" from Gust to our intermediate engineers, so we can continue to get features we need into these projects? We've straight-up asked Gust for help with this, and he flatly told us "go **** yourselves; introducing you to my buddies for my after-hours hobbies is not in my job description". I don't see anything posted in the mailing lists or GitHub pages indicating Gust is encouraging these teams to mistreat us, but I doubt such people would be so coarse to any newcomers engaging in the project unless they were instructed to act this way towards us.
We require features that can only be implemented by people with expertise (i.e. the maintainers). We can't fork the project for legal reasons I can't get into here. This mandate came from our legal team. What they say is basically marching orders for us (i.e. anything GPL is treated with contempt; Apache/BSD is a "maybe" if we're lucky).