A diamond ring loses a large amount of its value when you purchase it, because engagement rings don't really ever sell for what they bought them for. If he uses

A diamond ring loses a large amount of its value when you purchase it, because engagement rings don't really ever sell for what they bought them for. If he uses that MBP for work, it is a return on the investment as it allows him to earn income with it, eventually (hopefully) more than what he spent on it. The diamond just sits on her finger for bragging rights.