Easy to pay... impossible to get money back

I put a ring on layby at Sterns and the lady tod me all I needed was the layby card that has all my payment information. So I did not pay attention to where I kept my receipts and now because of personal reasons I do not need the ring anymore. I went to the store to ask for a refund but they tell me I must provide Proof that I was paying even though I have the first slip and the card with payment info They wanted to give me a voucher that forces me to buy from their store even though I do not need or want anything from their shop. I do not understand how I can be Forced to buy from a jewellery store, I have never been in a jewellery store and the only Reason that put me there was for the wedding that is no longer happening. I did not receive a product form sterns, but they refuse to give my money back.