Zuhayr bin Mua'wiya bin Hudayj زهير بن معاوية بن حديج · The Book of Zakah

"At the time of the Messenger of Allah, the people used to give as Sadaqatul Fitr a Sa' of barely or dates or rye or raising."

'Umar give a house in charity for the sake of Allah, the Mighty and Sublime, and he found it being offered for sale after that. He wanted to buy it, then he went to the Messenger of Allah and asked him about that. The Messenger of Allah said: "Do not take back what you have given in charity."