Is it permissible for the one who gave charity to buy it back from the one to whom he gave it, after giving it to him, in accordance with the noble Hadith: "Do not go back on your charity, even if he gives it to you for a dirham"?

The prohibition of buying back the charity (sadaqah) that was given for the sake of Allah is to prevent any attachment of the self to it. Buying it back indicates that the giver is still attached to it. Also, it is to prevent the seller from showing favoritism, which would result in a portion of his charity returning to him. Indeed, he who takes back his gift is like one who takes back his vomit.