Is the amount I received considered a gift, then later demanded back as a legitimate debt that must be repaid, knowing that the giver initially insisted it was a gift and that I intended to repay it when I was able?

If the donor explicitly states that the amount is a gift, then he is not permitted to demand it back, based on the Prophet, peace and blessings be upon him, who said: "The one who takes back his gift is like the one who takes back his vomit." The majority of scholars prohibit the donor from retracting his gift after the donee has taken possession of it. Therefore, you are not obligated to return the amount, and there is no sin upon you. However, if you wish to donate something from your wealth to him, there is no harm in that.