I'll-treatment by Decofurn

Went to Decofurn greenstone yesterday and bought 3 items which they said they were not available in store at the time so I need to pay for them so they can order them for me and I can collect at a later stage I did exactly that, I paid and they gave me a proof of payment which is a receipt (signed and stamped by Decofurn), today they calling to tell me that I did not pay so they won't be able to process my order, even when I Whatsapp them a proof of payment they gave me yesterday their store manager ( Ranga) still insist I did not do a payment so they won't process my order. He was very rude on the phone, telling me that what I have as a proof of payment is just a piece of paper there is nothing I can do with it