vODACOM THAT HAS BEEN USED

I bought a R 5 Vodacom on 29/09/2015 at 19:57 from this service station,when i loaded it,it said : voucher already used.<br> I immediately phoned vodacom,they then told me the voucher has been used.Vodacom sent me an sms on when and how. The sms states : <br> <br> Voucher sn number : *** . 06073***** first 5 numbers. Redemption date : 2015-09-13 18:31:32 Voucher Status: Used . SENDER:vODACOM.<br> HOW CAN THE SELL STUFF THAT HAS BEEN USED?