回複：Go back to China before receiving the Green Card

Can I still use my valid AP in hand?

-No.

 If I ask my friend to mail the green card to China, can I use that green card when I come back at the USA border in September?

-Yes. Another choice is you may print out the email and go to the USCIS local office to see whether you can get the green card stamp on your passport. If so, you can use this stamp to travel.