Before you leave the United States while waiting to be called in for and interview on your adjustment of status application (for a green card), you must apply for something called "Advance Parole," using Form I-131 issued by U.S. Citizenship and Immigration Services.
Hopefully, you already included this form with your adjustment of status packet. If not, you can submit it now, separately. You can avoid paying a separate fee with this application by submitting a copy of the Form I-797 receipt notice that you got from USCIS upon submitting your adjustment application.
If you don't submit this application before you leave the U.S., USCIS will consider your application to have been abandoned, and will stop processing it. You will then be denied U.S. entry unless you have some other form of visa.
An exception to the advance parole requirement exists, however, for holders of either an H-1, H-4, K-3, K-4, L-1, L-2, V-2, or V-3 visa. All of these allow for travel to and from the United States while your adjustment application is pending, as long as the visas are still valid and unexpired.
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