I'm not certain, but my guess is that the lower rate requires a Friday-night keep, and that requirement will likely be in the rental contract you sign. If you do not meet the requirements of the rate, then I think your rental rate will probably revert to the higher rate. Would you have a problem holding onto the car until Saturday?

If you are renting from and returning to the same (airport)location, have you considered putting in a bid on Priceline (or checking the rates on Hotwire) for this rental? That might solve the problem.