I have a Auto Loan Question?

So im a first time car buyer and i have a few questions. Say i was approved for a loan for 15,000 and the interest is 8 percent and APR is 0. If i was to buy a car that only cost 10,000.00 would the rest of whats on the loan check from the auto lender go back to them immediately and make my payments smaller or would it go to cash?