What is the ruling on using a cashback card where the user deposits their own money and spends from it without borrowing or delaying payment?

Firstly: If the cashback card is a prepaid card and operates within the limits of the deposited money without taking a loan from the bank, then there is no harm in dealing with it, with the exception of cards that offer gifts to their holders.

Secondly: The money in the card's account is considered a loan from the customer to the bank. It is forbidden for the bank (the borrower) to offer gifts to the customer (the lender), unless such gifts were a customary practice between them before the loan.