Cash back is given by many organisations but mainly at supermarkets. All it is really is a reduction in the cash held in the till and moving it onto the EFTPOS debit/credit card system. This saves counting the cash in the tills at the end of the day and then physically having to bank it, agianst having the same value paid direct into the bank account from the card machine operator 3/4 days later automatically.
How you record this really depends on how you already record all the till sales in your system.
Simply, the cash back just moves or transfers the till cash to the card side and then from the card to the bank account, instead of transferring the cash to the bank account when the cash is banked.
You could pass the cash back through a 'contra account' so to double check that all the cashback has been accounted for.
Hope this helps.