Don't worry, HDFC has a good track record for cashback fulfilment.
There is also thus condition:
Turn-around-time (TAT) committed with respect to posting of Voucher for a promotional campaign is only indicative & may vary from the actual date of posting; the Bank shall not be liable for any such delay. However, the Bank shall review the customer’s query & if eligible, shall ensure fulfillment is completed at the earliest. In case of delay in posting or non-posting of Voucher, if applicable for a promotional campaign beyond the committed TAT, customer is requested to contact the Bank; the Bank shall review the same & have the Voucher posted if the customer is eligible as per offer T&C.