Urban legend that some sales people repeat, but when you actually try to do it, they tell you it cannot be done. I was told the same thing on one of my Tesla purchases, then I brought not only a quote, but an actual offer to buy my trade-in at a higher price if Tesla would only trade it in so I could get the 10% sales tax credit, and they can just sell-through to the dealer. They neither upped their offer or do the sell-through, my best guess is because they knew the difference was less ~10% so it would have been a wash for me to sell it to the dealer instead of Tesla, while more convenient to sell to Tesla because I can just bring it in when picking up a new one. That was before Elon's laser focus on profitability, so today it would be even less likely for Tesla to up their price.
PS> The funny thing was the excuses I got. First I was told any offer would be ok, then they told me I need an offer from an actual NADA registered dealer (so no offers from non-dealers), then when I got one, they told me they still can't do it. So it seems they tell you want you want to hear, then make up excuses (like Elon, when he sold me a 700hp P85D which was supposed to have an 85KWh battery and have summon find me anywhere on private property for example, or like when he told me that if I buy a Tesla I will always get same or better Tesla loaner when I bring it to service - all complete and total BS purely just to make a sale). Beware of anything Tesla sales and marketing says.
