We want to return the phone

My sister bought a phone (Huawei P30 Pro) on the 30/01/2020. She was not told that vodacom doesn't activate dual sim. We realized that the sim2 was not recognized during the weekend. Sunday we went back to Vodacom (Mall of Africa, where she bought the phone) the sales guy told us that Vodacom doesn't activate dual sim. The sales guy didn't even give my sister a receipt, he gave her insurance documents only. I called the manager (Gordan) and asked him, Why was she not told before buying the phone that it doesn't take the second sim and we only figured it out after opening it. My sister is not happy and she wants to return the phone. Gordan said, she can only return it if it is on contract. Since my sister paid it once off she can't return it. How so? I don't understand! it is still within 7 days.