CASHIER SOLD ME A PHONE I DIDN'T WANT

Seven days ago my mom bought a phone at Total Sports Middelburg. She told the cashier that she wanted a phone with dual sim but she gave her a phone with only one sim card. She only noticed the next day when she asked my sister to set up the phone for her. When she went to the store they told her that they will not refund or exchange the phone for her. The cashier said that she forgot what my mother said. It's not my mother's fault that she forgot how to do her job. She gave them back the phone but she didn't get any money back. We want our money back.