This transaction is haraam, because it involves a commitment to pay ribaa (interest) if one does not pay within a certain time. This is an invalid commitment even if a person believes or thinks it most likely that he will pay before the time is up, because things may change and he may not be able to fulfil the commitment. This is a matter which has to do with the future and man does not know what will happen to him in the future. So this kind of transaction is haraam. And Allaah knows best.