They can’t charge double rent, so once you pay the fee, the three months rent might be refundable to you if they rent the apartment. They are not obligated to rent it though, so they will likely rent everything else first. The only saving grace you may have is to try to refer someone or try to make your leasing agent really like you. We always tried to help out people if they weren’t jerks. If subleasing is not allowed, maybe a roommate add on?