Good tip on cancelling within 24 hours of booking! I wonder if it works only in US or in other destinations too.

In my case, I have dealt with Sixt via Twitter support, it was not straightforward to be honest and I did not get any price difference refunded.

What I did do though, was the booking change, change the car category to a higher class, the amount for higher class SUV was the same as previous amount paid for lower class vehicle, so I had to pay only 30EUR change fee.

So to sum up my situation, if you paid 1000 EUR for a pre-paid rental with Sixt in Europe and the prices have gone down dramatically afterwards, you cannot avoid the cancellation fees (that are worth 3 days of rental fee), but you can use this 1000 EUR as a paid deposit to book a better car under new rates and your new total will be 1030 EUR + any difference in price if any. In my case I managed to rebook from FFMR to PFAR just paying the 30 EUR fee. Not the best scenario possible since no money saved but at least you can get something more for your paid amount.