Is it permissible to give zakat to a poor married person as a gift (naqūṭ), knowing that it is customary for the recipient to return what was given to him on an occasion pertaining to the giver?

The poor are one of the categories for receiving zakat mentioned in the Quran. If the person mentioned is poor, it is permissible to give him zakat at any time. The fact that this person might reward the one who gave him zakat on another occasion does not prevent giving him zakat, as long as he remains poor.