While it's recommend that you wait until you get your actual plastic Green Card, which you should get fairly quickly after your arrival to the U.S., because you stated that the language in your passport shows that your I-551 stamp in your passport is valid for one (1) year, you can leave the U.S. before receiving your actual plastic Green Card. The I-551 stamp in your passport is currently your Green Card.
However, when you come to the U.S. and depart, please remember that you should not remain outside of the U.S. for six (6) months or more anyways, as that will lead to a presumption that you have abandoned your Green Card status -- you definitely don't want that.
Please make sure you consider all of this as you make your plans for the next year.
If you leave the US, how are you planning to get back in without your green card? There is a slight possibility that you receive it before you have to leave. As a word of caution, be careful how much time you spend outside the US, because you can lose your residency if you stay out too long.
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It would be advisable for you to wait until you actually have the green card. Additionally, if you intend to depart for six months or more, you will need to get a re-entry permit.
J Charles Ferrari
Eng & Nishimura
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As others have said, it is preferable to wait until you have the plastic card. Also, while you can leave and reenter using the stamp in your passport, you should realize that the green card requires that you be a RESIDENT of the U.S. This means you must file US taxes on ALL income from ANY country, even if you spend part of the year outside the U.S., and otherwise demonstrate substantial ties to the U.S. As a colleague said, if you need to depart the U.S. for more than six months, you should file for a reentry permit. You should consult with an immigration attorney regarding the requirement to maintain residence, as well as physical presence and other requirements for U.S. citizenship down the road.
This general information does not establish any attorney-client relationship. There may well be factors not mentioned in the question which could and should be addressed in an attorney consultation.