"Change my mind about trade in my MacBook Pro: Hey there! I bought a Mac mini and I should send my old MacBook Pro for trade in value. Why if I changed my mind, and just want to keep the MacBook Pro too? Is there something that I need to do or notify?"
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Getting Mac Back After Sending Mac In:
No. Once it's been handed over, you cannot retrieve it back. However, keeping it, the trade-in would not even take place.
IMPORTANT; Prior to trading in, perform as inserted here, including the Time Machine Backup creation and iCloud logout: What to do Before you Sell, Give Away, Trade in, or Recycle your Mac - Apple Support