Most likely, they cannot prevent you from leaving

Laws vary from city to city. However, it seems that if your car is NOT hooked yet, you can always go for free.

Here is a "exceptionally good" law:

Section 353-65 (E) of City Code:

“Except for tows authorized by the Police Department, it shall be unlawful for a towing company operator to tow a vehicle if the owner or operator of the vehicle appears at the scene, requests the towing company operator not to tow the vehicle and is willing and able to correct the condition warranting the tow. The towing company operator shall not tow the vehicle even if the vehicle is connected to the tow truck, provided that the tow truck is not yet in motion. Except as indicated below, the tow truck operator shall not be permitted to charge any fee to the vehicle owner or operator even if the tow truck operator has connected the vehicle to the tow truck, unless the owner or operator is the one who requested the towing services. "