1. When accepting payment by debit card, merchants pay a fixed commission fee (as opposed to a percentage) to their bank or merchant service provider. (This is because the commission paid by the merchant for accepting debit cards, unlike credit cards, does not need to fund interest free credit or other incentives).
2. Accepting payments in cash can be costly for merchants, given that many British banks charge around 0.5% for depositing cash into a business bank account, along with the costs of transporting and insuring the cash.
The combination of these two factors means that the retailer can save money by offering the cashback service. It does not cost the retailer more in commission to add cashback to a debit card purchase, but in the process of giving cashback, the retailer can "offload" cash which they would otherwise have to pay to deposit at the bank.
Merchants do not offer cashback on payments by credit card because they would pay a percentage commission on the additional cash amount to their bank or merchant service provider.
Some vendors enforce a minimum purchase amount or add a fixed fee when providing cashback to a customer.
In many cases, retailers require customers to initial the cashback entry on the till receipt to confirm they have received the cash. This system is used to prevent cashiers surreptitiously adding cashback amounts to a transaction and keeping the money for themselves (or accusations of same), but more importantly, to ensure that customers cannot return to the store with allegations that the attendant "forgot" to hand over the requested cash.
Cashback can have benefits for the customer in many scenarios. In locations where there are no cash machines nearby, or the nearby machines are out of order or empty, a local retailer may be able to supply the required cash instead and to offer more flexibility in note denominations. Sometimes it is simply more convenient to combine the transactions at the retailer and ATM into a single cashback transaction with the retailer.
Additionally, although fees for debit card ATM usage are very rare in countries such as the