Bad service

Tonga mall Achermans ****** me my money.I labyed a cellphone (sumsang j4 core)which cost R2000 on 16 july 2019 and deposited R1000 on a gift card.Few minutes later the cashier (Madona) confirmed that the money was deposited successfully into the gift card.So last month on 19 August 2019 when I returned to the store to pay the outstanding balance they told me that the gift card have 20cents instead of the R1000 that I deposited and told me that the card was used to purchase some items in Soshanguve a place I don't even know where it is located.I tried explaining to the cashier's but they don't want to listen and help me.