I don’t see how they can insist you deliver your trade-in to them with the same mileage since they know you are not delivering it until the deal is complete and your new vehicle is delivered. If they try something like that – and your used car is in the same condition as it was with the expected number of add’l miles, that’s a reasonable expectation – you always have the option of refusing delivery and forfeiting your $500. You might be able to get the $500 back in small claims court in that event.
I doubt that will happen though. At least if you play your cards right. What I’d do is constantly remind the staff there at every opportunity that you appreciate their help in this purchase and based upon how you’ve been treated you hope to purchase another car from them, next time you need to buy a new car. And if they still try to upsell you other stuff, don’t get overly offended, that’s them just doing what they are told to do by the dealership manager. Just tell them the cost of the purchase has put your bank account balance so low that if you buy anything else you’ll overdraw the account, so while you are happy to consider their ideas, you’ll have to postpone other purchases for now.
When I purchased my Corolla from a dealership – this was 20 years ago or more – I started with this rhetoric right from the start. I kept repeating to the salesman “Wow! It costs $11,500? This is a major purchase for me. I’m broke after this. I’m not able to afford anything else”. I must have said the same thing 10 times over the course of a couple weeks of negotiations, as the staff there were searching for a car with the options I wanted, which was a manual transmission and no other options. (I did have to agree with one other option, the radio. Ok, I made a compromise! )
Anyway by the time the deal was struck and the paperwork was written up, the salesman said to me “George, we usually ask our customers at this time if they’d like to consider some other purchases like extended warranty, undercoating, paint protection, etc, but I’ve left those off the paperwork because of your earlier comments. Is that ok?” … lol …