On what grounds can a pending severance offer be rescinded?

I was handed a signed separation agreement stating that I had 21 days to read, consider and return it. On day 16, the CEO sent an email rescinding the agreement, stating an unexpected and negative financial report on the company from their outside CFO would require laying off several other people. And that they could no longer afford to honor my original agreement.

Prior to receiving the rescinding email, I had some insurance questions about my original agreement. At one point, the VP sent an email saying 'prior to signing the agreement, please call me next Wednesday to discuss'. The Wednesday he referred to was 5 days later, and 2 days AFTER my original offer was rescinded. I believe this may have been done to stall until they could rescind my offer.

The rescinding email had no text, just 2 attachments. I viewed the email, but I opened neither attachment, as I knew it was different offer. The email was traced and they claim to know I read the second agreement. I know it is possible to verify that an email was viewed.

I signed and FedEx'd the original agreement that same day, but it was 'stamped' and hour after I viewed the CEO's email. Can I press them to honor the original agreement, and does simply viewing an email actually give them the right to subvert their first offer?