But it's not an exploit. Both parties are benefiting. It's like he's handing you cash, and you want it out of the goodness of his heart or for a small fee? But maybe you should meet him closer to that value he is giving you, out of the goodness of your heart. How would you even pick who to pass the deal onto, from a bunch of strangers? You are directly getting $15-$20K in actual value, that will go to you either now and/or when you sell the car eventually as you'll be well ahead of everyone else's depreciation curve.
Which is it, money doesn't matter or it does? If it doesn't, then you should have no problem compensating him fairly for the value you are getting. If any of us had a Model S right now available just picked up, we could sell it for $95K-$100K+. You wouldn't ask the person to take $80K out of the goodness of their heart? Why, because he has a reservation about to be delivered? Which is arguably a better value to the buyer who now is the first owner, but they're now worthy of a massive discount? It's good for you that you passed along your tickets, but that's your choice. I bought my house 4 years ago and am about to move, I'm not going to sell it for what I paid to not "hustle" the next guy. I'm going to sell it at the market price. Why is it that in certain situations, people expect others to give up a valuable asset for free? (in this case, his discount that he earned by ordering well in advance).
I also have an $80K res I no longer want, as I found out my wife is pregnant, need a new house, and with 2 babies I think the Model X is more suited to our needs. Now if I want that, I have to pay $15K more than the "old price". Why can't I offset that by capturing value from my current reservation? By your logic I should give it away free or cheap and just spend an extra $15K on the Model X with no offset? Why can't I make it so that I get a Model X at the old price by capturing value of my Model S? I have a family to feed/shelter/educate, I can't afford to give out $15-20K. And assuming OP can't either.
The most altruistic outcome I'd consider being "nice" is splitting the difference of the discount. But even then the buyer is capturing more value as they are saving on sales tax and fees, and again getting benefit of any extra "value" in the market price over sticker in today's environment. And the seller is essentially selling the right to a $95-$100K asset for $80K plus $7,500. I still think there should be some discount because of the work the seller is saved by not having to take delivery, market, sell, etc. But not $10K.