If someone promised a poor person something, and what he promised was destroyed, must he buy it for him? And is it permissible for him to buy him something better than what he promised?

The forgetful person is excused. You are not required to buy meat to give to someone you promised a share to but forgot to fulfill the promise, because the Prophet (peace and blessings be upon him) said: "Indeed, Allah has pardoned for my Ummah mistakes, forgetfulness, and what they are forced to do." Fulfilling a promise is considered commendable by the majority of scholars. There is no harm in buying meat that is better and more than what you promised, as an act of charity and benevolence.