Again falling back on contract law, but an offer remains open until its rescinded. If a deal is no longer on the table, do the other GM the courtesy of knowing that, especially if it was 5 minutes ago that you made the offer.
So basically, if you make an offer and don't rescind it, it could still be accepted say, thirty years later, and you would have to agree to it. (real life hehe)
I guess that's kind of weird, but I feel at some point, the contract should automatically become VOIDED for simply being too outdated.
A twelve-hour acceptance period, or the deal goes away