Buying back a gift from the one to whom it was given

It is not permissible for the giver to buy the thing he gave
to his brother. It was narrated that ‘Umar (may Allaah be pleased with him)
said: I gave a horse to someone (to ride in jihad) for the sake of Allaah,
and its owner neglected it. I thought that he would sell it cheaply, so I
asked the Messenger of Allaah (peace and blessings of Allaah be upon
him) about that, and he said: “Do not buy it, even if he gives it to you for
a dirham, for the one who takes back his charity is like a dog that goes
back to its vomit.” 

And Allaah is the Source of strength.