If that's a corporate code that requires employee ID, they can refuse to rent to you, of course. If they refuse, they obviously have to refund the money you prepaid.

Here are the down sides I can see: you don't have an employee ID and you don't even know the name of the company or where the code is from (I haven't figured that out yet - I'll keep looking in my spare time). In addition, the fact that the discount is so large will likely call attention to your rental.

However, the very worst that can happen is that they refuse you the rental, refund your pre-payment, and try to charge you the walk-up rate. If they accuse you of wrongdoing, simply claim ignorance - say you didn't realize this was a code that only certain people are supposed to use.

In this case, having a back-up reservation with another agency is essential (obviously don't prepay for the backup). Then, if the worst happens, simply thank the Avis agent for his or her time, and make your way to Hertz or wherever you made the backup reservation.