I also read about "the contract he signed", it just doesn't make sense to then have a dispute as whether there was an early termination clause in it, or not. You'd merely have to look.
IF a contract is written, but then something additional is agreed verbally, then technically it counts, but how would you prove it?
The point in similar circumstances is an employer has no choice but to accept an existing contract warts and all, but that (if written) would be the extent of it for both parties. If some time later, the employer claims there's an additional verbal wart, then how would he know? And how would he prove it? Call the previous employers who alleged it as witnesses?
Would be interesting to know how those general observations fit the facts of this case, but I doubt we'll find out as I expect a deal surely must be done.