Unlikely case of non admittance into USA?

My green card has an expiry date that is past now. Because of unusual cirstances, I have been out of the country for the last 3 years. Based on my special cirstances, the emby has issued me a transportation letter, however. In the unlikely case of not being admitted at the POE, into the USA, will I immeditely have to buy a return ticket on the next flight and fly back and is that the worst that can happen?