the store I work at is closing and they are giving the workers severance pay as long as we stay til the closing date.

I worked at Bottom Dollar for a year and on November 5, 2014 they called everyone together to announce that the store would be closing due to the company selling out to their competitors. This being the case they would have to give us, the employees, severance pay. They had us sign a contract that stated that if we leave the job or are fired before the store closing date January, 5 2015 then we would not receive severance pay. But now the store is staying open longer and I cannot stay past January 5th because I have an obligation that requires me to leave. My question is can they do this? Even though the contract we signed says that we had to stay until the store closed on January 5th. Since the store went back on their word do I have to stay there to receive my severance pay, can I leave and still get my severance pay, or can I take legal action against them?