There are some caveats. The MacBook Air you trade in needs to be in working condition (no water damage, no cracked screen), the trade-in must take place at a Microsoft store, and the money you receive for the trade can only be used toward the purchase of a Surface Pro 3.
The trade-in offer expires on July 31st and we should point out that Microsoft has used that old trick of saying that the trade will return "up to" $650. If your MacBook Air has an intact screen and no water damage, you still might not receive the entire $650. Deep in the fine print, Microsoft writes that when it comes to the amount received for the MacBook Air, "Condition, documentation and accessories may affect value."
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source: Microsoft via Slate, Geek