I'll elaborate since it is relevant to the discussion - I'm passing through land ports from Western Canada, which might have an impact here, but CBP simply does not want to take my green card, and I've moved back home. I work out of Canada, I've declared that I wish to return my green card, and they simply aren't having it. I think I've tried three times already. First two times the dudes at the window just told me no, wait, hang onto it, and they basically advised me to use it as a fast pass for the border. The third time they sent me to secondary, where a supervisor told me not to turn it in. Honestly, it's too intimidating to proceed at that point so I always go "SURE! :D" but like, I don't like being between statuses and I don't intend on moving back to America. The like, supervisor said "You have until the expiration date on this card to make up your mind." Honestly, I believe that if I told them "I'm working and living in Canada for a few years, and I plan on returning to America" they'd let me keep it the entire time and move back when I wanted to.
With that said, I'm not sure if airports are a different culture or not when it comes to CBP. It's also worth mentioning that I've never been out of the states for a full year, because we visit my inlaws very frequently.
Also - try calling them directly. Explain the situation and they'll likely give you an understanding of how militant they'll be about everything. I call them for all sorts of stuff if I have questions and they are generally pretty helpful.