Giver buying the gift from the person to whom it was given

Praise be to Allah.

It is not permissible for the one who gave the gift 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 use in jihad) for the sake of Allaah, and its owner neglected it. I thought that he would sell it  for a cheap price, and I asked the Messenger of Allaah (peace and blessings of Allaah be upon him) about that. He said, “Do not buy it from him even if he sells it to you for a dirham, for the one who takes back his charity is like the dog that goes back to its own vomit.” Narrated by al-Bukhaari, 2623; Muslim, 1620. 

And Allaah is the Source of strength.