Are you really that distrusting? Maybe you have had bad experience with retail outlets, or maybe the suing culture of N.America. Either way, people do look out for others, and certain statements in a policy are more guidelines rather than rules. I have worked in retail (in the UK that is) and the manager is able to make a promise, he is the one running the store - if the store does not benefit from it and loose money it's their job on the line.
But in this case, it doesn't even make sense. Letting someone use and then return merchandise sometime in the future? What if it is 6 months? A year? Would you allow someone to do that if you ran a business?
And the situation may be more complicated than that. What is the new Mac Pro's cost less for a comparable configuration? The OP will then want a refund, but that wasn't in the verbal agreement. The verbal agreement was for an exchange.
What if the new Mac Pro's cost more for a comparable configuration? The store will likely want more money and since they have the power in the situation they can demand it or simply make you keep the one you have.
The manager could have told him he would hand deliver it, set it up and make him dinner because at the time it required no action.
It isn't about distrust, it is about doing what makes sense and this doesn't make sense. If the OP did not get it in writing, you would have to a fool to believe this would play out the way it has been described. That isn't to say it won't happen. But what is it they say? If it sounds too good to be true...