Nissan Motor Acceptance Corporation Monthly payments not recognized. Phoenix, Arizona

End of February of 2012 I bought a brand new 2012 Nissan Rogue from a dealership in Richmond, Virginia called Sheehy Nissan and was told once the financing was accepted, that I would be paying $429.53 a month. 
Everything was fine til on June 23rd, I paid twice $429. 53 ($859.06) covering in advance, the month of July and August, as I was going to travel abroad, and asked the company not to deposit the check for August, until the very end of July..which of course was not taken into account..I found out that both check were deposited at the same time..!!

At the time of writing these two checks..the date on the July statement was July 13th, and the payoff amount showing on that statement showed $27,268.59, so logically, I assumed that by paying in advance July and August, the August interests would not be debited, as I was paying this in advance, but even if they had not debited that amount from the August statement I still would have gotten the $ 859.06 (2 x $429.53) credited to my account..and I should have received an August statement showing these two payments...roughly a dollar amount of 26,000 + Interests.

To my surprise; not only was the amount not showing (previous payoff amounts always showed the payment of $429.53  being deducted from the month's payoff total + interests)....but I got a statement dated August 13th with a payoff amount increased to: $27,312.40..

Where are my payments of $859.06???? why was I charged for August interests, when the mount paid was paid 1/1/2 month ahead of time????

Where did my money go...why do they insist that there were interests involved, when previous amounts paid would show the monthly payments of $ 429.53 deducted from the total payoff amount every month..except NOW!!