Well... first of all, you shouldn't do the cash deal in the first place. Normally when buying used cars, instead of saving the taxes, you should try to make it in some extra features like extended warranty, or a few oil change, or engine/transmission flush.
For your case, I strongly recommend you to exchange the car. Why? Because you will have a hard time selling the car later, not to mention you will worry about the condition of the car because its mileage was altered (if it is true).
I will even suggest you try to get a higher price car (as a bait) to negotiate for a better deal. This way the dealer will be more willing to negotiate because they can make more money, versus they know they already pocket the $9800 and they won't eager to talk. It is just my suggestion. Maybe financially you aren't allow to pay more. You make the call.
Make sure you get everything on paper this time. The dealer won't cheat on you because if they were accused of alter odometer, they can lose their license.
Oh... one more thing. You mentioned the odometer was adjusted to 14,000? and you said you got the number from Aircare? Well, it is also possible that AirCare made a typo and it suppose be 140,000? I know it still was altered, but maybe not as bad as you think?