Probationary employees are fired afaik without any severance, as they are not yet covered by worker protections.

Even those who do accept the “buyout” may be unpleasantly surprised in March with the looming government shutdown and corresponding lapse in appropriations.

https://apnews.com/article/trump-firing-probation-workforce-...

The deal says that if they have no legal standing to make any challenges once signing. If they decide to not pay them, they have signed away any legal remedy

Only if you believe a non-official email that involves signing away all legal rights to any recourse you might take against them both before and after leaving, which includes your severance pay which nobody has authorized or found a source for.