Timing belt

Purchased a vehicle from a private owner about last week and he had the timing belt changed August of last year at a dealership in the area. The timing belt is still under warranty my question to you if I am hearing a whining noise when I hit the gas pedal could that be the one of timing belt tension gone bad? I called the dealership and they stated that since the previous owner had the work done on it and I'm the new owner that I would need to pay 105.00 diagnostic charge and if it was their fault they would fix it and give back my money if it's not their fault that they would put the 105.00 toward the service to fix the car if I didn't want them to fix it they would keep my money. My question to you is what should I do because I do not have 105.00 just to be giving away.