Wasalam
May Allah bless you.
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Paying someone an amount of money so that they offer you a cashback amount is not permissible. See point 3 in the answer found from the link: Permissibility of different types of cashbacks - #2 by Mufti_Billal
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In regards to the gift cards. it is not permissible to purchase a gift card at a discounted value. It follows the same principle as above i.e. the prohibition of selling money for a different amount.
And Allah knows best!