Good question. I recently had a client with a similar experience. A stamp should be enough. If you would feel more comfortable with having a green card in hand, you can call Homeland Security and ask where the card is. Nicely explain that you are leaving the country and would like to know when you can receive. But remember this is a bureaucracy and it may take awhile to receive the card.
A valid stamp is enough.
J Charles Ferrari Eng & Nishimura 213.622.2255 The statement above is general in nature, as not all the facts are known. You should retain an attorney to review all the facts specific to your case in order to receive advise specific to your case. The statement above does not create an attorney/client relationship.