Ineffective systems

I made a purchase at Woolworths in Woodlands Mall on 11 July. I paid with my bank debit card and almost immediately received an SMS reflecting the transaction. The cashier then informed me that the till had failed to generate a pay slip. After investigating the reason for this, I was informed that the transaction did not reflect on their system. The manager told me the transaction had to be reversed and another transaction be done and that the overpayment would be refunded to me within 24 hours. I refused to do so, due to the risk that the second transaction could also fail - which indeed happened to a client who lodged a similar complaint about Woolworths on this website on 13 July. Up to now, I haven't received the refund, despite the fact that the manager took my number. I have proof of payment, but still they refuse to regard it as a valid transaction. It's not clear why customers must bear the risk and suffering for their system failures.