The pension fund can be $0.00, the amount of money Boston owes it's retirees has not changed.

Beyond that, I doubt there is any scheme you can come up with to try to implement what you're dreaming of that would pass a court. An employee signing a contract doesn't make illegal provisions valid.

There's a lot you can't do with compensation, and "taking money you've already paid away from one worker because of the actions of another" is usually right up at the top of that list, no matter how you want to phrase it.

As a basic example: Employee 1 crashes a company truck. Maybe he runs some people over in the process. Company is out $1m. Can the company come back and say to the other employees + former employees with retirement accounts "we're going to need each of you to return $10,000 from what we've paid you in the past to pay for it?" - obviously not, it's the company's problem. If they've got a strong enough case they can maybe sue Employee 1 for negligence although that probably won't come up with $1m.

But nothing they can write into their employment contract can force people who had nothing to do with it to pay for the company's problem.

The city of Boston is the "company" here.