It is pretty standard practice to require a signature at the very least when issuing a refund to a customer. And if the cashier had tendered an amount into the till, then issuing a refund is the only way to return money out of the till as you can only 'void' while the transaction is still live.
But I do agree that to ask for all of your details is a little OTT as it was a price discrepancy and not a return. Your initials on the refund slip should suffice, just on order to cover the money out of the till. We have had situations where this policy was not in place whereby operators were issuing bogus refunds and pocketing the money.