Dasparate to do an exchange

I bought fascia board from Builders Lifestyle Centre last Saturday 12 June. My mum's builder confirmed in the week that the board we bought was wrong. We decided we would return it today 19 June, but I realised that I misplaced the slip. My mum remembered that I had sent a copy to her via whatsapp because I had forgot to leave the original with her. We went to builders today to try and exchange using that printed copy but they did not agree. We are in desperate need to exchange that board because the correct board needs to cover up an opening at my mum's house. Mum is a pensioner and I also do not have money to buy another board for her. Is there another way for us to prove that the copy we have was not a **** or ***** because that is what they kept insisting on at the shop. I am desperate for my mum, she is a pensioner and she might have heard the builder wrong on the name of the board that must be bought ?