Dollar Rent a car DTG operations, Inc Nickle and dimeing early vehicle parking fee Detroit Michigan

 You seem to be leaving some thing out.  Such as how/when you dropped the car off.  It is very unlikely that your car rental was for example $100 and by you returning it early they billed you another $20.

It sounds more like one of a couple cases.  Perhaps you rented the car under a weekly rental rate and probably dropped it off 1 day early.  Well the fact is the weekly rate is already discount off of their daily rate.  Where after 4-5 days it is usually cheaper to just rent it for a week.  But if you return it early you don't get any money back, because if they converted you to the daily rate you would be paying significantly more.

Perhaps you just dropped the car off after hours or on a day that they were closed.  Where unfortunatly a)There is no proof that you did this, and b)When you originally rented the car you were not scheduled to drop off the car.

Of course if it is something else perhaps putting the entire situation may help.