Praise be to Allah.
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.