Younker Nissan not willing to correct a $5800 mistake Renton Washington

I went to Younker Nissan to lease a 2015 leaf SV (LED + QC) on 9/19/2015. I was greeted well and the negotiation went well as well.
I was trading in a 2007 Nissan Altima 2.5S for which they agreed to pay $5800...
The negotiated price on the lease was $9684 for 36 months. So the net I was going to pay out of pocket was $9684-$5800 = $3884.
I have a copy of the worksheet on which this is explicitly listed.
They drew up the paper work and I signed it.

On the way back home(about 20 min after I left the dealership) it occurred to me there was a mistake and the $5800 was applied in a way that would make the total out of pocket $9684.
So I called them and the lady who sold me the car assured me that on the first monthly statement I will see a $5800 statement credit.
She repeatedly assured me that my monthly payment amount is still going to be the same but I will make a total of around 12 payments after which the lease payments would be satisfied and I no longer make any payments.
With this assurance, I went home.

A few days ago, I got the first statement and see that there was no statement credit.
So, I went back to Younker Nissan on 10/11/15 to ask them why this is the case.
One of the finance managers took a look and told me everything was fine on the lease and that I agreed to pay a total of $9684.
I showed him to worksheet that we originally used to negotiate and he refused to look at it saying it is not a legal document.
So, I went to one of the managers and she said the same thing that since I signed the lease she doesn't care what was agreed on. After some persuasion she looked at the worksheet and I was dumbfound by her response.
She said all that piece of paper said was the subtraction of 9000 with 6000 results in 3000. She essentially gave no weight to the numbers.
Another manager, while a bit more polite, explained the same thing and told me that the manager that made the deal was not in office and so he asked me to come back on 10/13/15 at 2pm.

I went back at the specified time and spoke with the manager that made the deal. He recollected the deal. I explained to him that there was a mistake and that it should be corrected.
He initially suggested that he will have to pull the whole deal and redo it. Which would have been the right thing to do.
After some more thought and after going over the lease a couple more times, he reversed his statement and told me "I made money". And that he is not willing to do anything about correcting the mistake since I signed the lease.
He also gave me an example that "This pen costs $5, if I said it was $20...".
It was very evident he was not willing to do anything about the mistake. I left.