Wrong prepaid voucher/Manger said we must sell it ourselves

Good morning my sister went to pep cell Balfourmall in Highlands north Johannesburg, to buy vodacom airtime and was given an MTN voucher by the lady who was assisting her and talking to her friend at the same time, when she got home she realizes that it's the wrong airtime and the next day she took it back they told her she has to sell that airtime herself because they don't take the voucher back, according to my old knowledge if a person was given the wrong airtime the store phones the service provider to validate if the airtime was used or not then take the right steps of refunding the customer or give her the right voucher please help; the store managers name is : Tumi Molehe (if this is even his right name)