Can you add miles to an odometer? And how do i have it corrected?

What is your question?

I took my car for an oil change ($73 dollar oil change at that) and there was 7000 miles added to my car when I left. Is this possible, why would someone add miles? The guy did want to buy my car pretty badly. If it can be altered, can I see that it's been altered in the PCM or anywhere else? I log my miles often, I know for certain miles were added. I'm just not sure how or why. Any info is greatly appreciated. Thank you Andrea

How long has your 2010 Mercury Grand Marquis had this problem?

Directly after I got in my car from the oil change.