Is the amount returned by the bank at the end of the month, which is conditional upon spending a certain amount monthly, considered a discount on purchases or usury (riba) on the remaining funds in the card, and is it permissible to spend it on the poor if it is usury?

The percentage that the bank adds to your account in exchange for your card balance is considered unlawful usurious interest, because the card balance is construed as a loan from you to the bank. These funds must be disposed of by giving them to the poor and needy, not as charity, for unlawful money cannot be owned. If the person is poor, he may take from this money an amount equal to his needs.