So in summary, 'legally' they could fire you if they wanted to.
However, if you have 'proof' that a manager gave the approval to do that, then escalate up through the management chain. Ask for a review from a 2nd line manager above who gave you the approval and show them your proof. If you have no proof that your immediate manager gave the approval, then you are most likely out of luck and you've learned a lesson to keep documentation in situations like that.
With proof, they may still fire you anyhow, as they'll look at other performance as well to see if you are an employee that they want to keep around, particularly given the economy downturn. But, if this was the only problem with an otherwise exemplary performance, with proof they could reverse the previous termination decision.
Good luck.