Check around the shop area and nearby streets for fresh black marks on asphalt and check shops near those marks for witnesses. Take pics and get names of witnesses and show them pics of your car. If he burnt that much gas, he likely made a lot of noise and laid some rubber. Find out where he lives and check his neighbourhood.
Record any discussions you have at the shop or elsewhere on your phone. Take pics of any oil spots where the car was parked at the shop.
It will be your word against the shops, if they are dishonest , you need evidence.
Good luck. That is a sin and a crime.