All perfect praise be to Allaah, The Lord of the worlds. I testify that there is none worthy of worship except Allaah and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and Messenger.
We do not see any harm in benefiting from the cashback offer and using the coupon code to get the extra free credit.
The resolution of the Islamic Fiqh Academy reads, "If the discount cards are issued free of charge, then it is legislatively allowable to issue and use them because they are considered promises of donations or gifts."
Allaah knows best.