Legal? Sign amended severance package or recieve nothing but already have fully executed agreement

I was fired on Feb 23rd and given a severance package that need to be signed and returned no later than March 2nd. I sgined and returned the fully executed document on Feb 25th. On March 2nd, I recieved an amended agreement that said there was a clerical error in the first agreement and lowered my serveance pay form three month serverance at 22K so 3 months severance at 16K. Even though the amended document is correct, can the company still be held to the orginal fully executed document with the 22K amount since it was signed and counter signed. Also, when I called to discuss I was told to sign the new agreement or I would not recieve anything. Can they do that?