lay-by Cancellation

I did a Lay-by at shoe city and I was not told that I have to keep my first slip in case of cancellation . I have been paying the lay-by using my Id number but now that I want to cancel the lay-by um told that i need to present the slip of my first deposit .I fail to understand why I can not cancel the lay-by with my id number as I have been paying using it and get my cash back instead of getting a voucher. I was told that even when I pay the full amount for the lay-by and do the refund I will not be given cash back but a voucher as I do not have the deposit slip , This sooooo disappointing and frustrating .