This is just what Musk did when he took over Twitter, paying an extra severance; only the first 20% got paid their severance. He closed the offer and stiffed the rest once the resignation notices started flooding in as the weeks went on and he started fucking things up.
As for the Feds, an employee either resigns and leaves with only everything owed up to two weeks out or retires, and it takes 3 -6 months to complete the retirement process.
If they're expecting anywhere from 6000 - or 60k - to resign with 7 months severance, the amount of money that much severance needs congressional approval.
Figure an average base hourly rate for, say, a 10 year GS 12 at around $41 an hour x 160 hours (the standard month) x 7 and you get a severance package of $45,920.
5000 across the Government decide to take the "offer". That ends up with the expected average estimate of around $229,600,000...around $300 million. That's pretty much the average point, but chances are, you're going to be seeing the GS 13's through 15's with 20 years quit; people whose base is closer to $50 or $60 an hour. So it's probably closer to almost half a billion put aside for severance, plus whatever accrued leave they have.
That sort of money takes Congressional approval.
And I don't see the Tax Cut at all costs "Freedom Caucus" putting up with spending that much money on "quitters".