So I called Hyundai Leasing and they told me the charge was for the two front tires. I will try to fight this due to many issues I had with the car and there are case numbers to prove this. BUT they told me I had to call Customer Care as they are not the ones who would issue any credit. After a very long conversation with Customer Care, they told me they cannot issue it, only Leasing can..!! It's a ping-pong game.
in all honesty I figured as much when you said you only replaced the rears.
did the dealer notate the wear on the front tires? I think that would be your only "out" the lease is very clear as to what is required when the vehicle is returned, and what level the tires tread depth has to be at.. It's likely to cost you far more in arbitration fees, attorney fees, court costs etc, than the $400, that frankly you would have paid for 2 tires anyway.
The leasing company isn't going to care about any issues the car had if they were covered under warranty, that is what the warranty is for, so unless you had out of pocket expenses (rental cars don't count) any arbitrator is going to find against you, I speak from experience, and if you read the paperwork, if you lose arbitration, you will likely have to pay for the arbitrator and any legal fees by Hyundai.
As you have returned the car, I am guessing you opted not to get another Hyundai? If you have washed your hands of the brand, and it sounds like you have, then if it were me I would pay the $400 and move on, as I said you would have paid that anyway had you replaced the tires yourself.
I realize that $400 is a lot of money, but to me the time and hassle of dealing with the Leasing Dept, Customer Service and all the stress and headache isn't worth it. It takes me just over 4 hours to make $400, if dealing with Hyundai takes more time than that, my time is more valuable and it isn't worth it to me..