"I heard 'Umar say: 'I gave a horse to someone to ride in the cause of Allah, the Mighty and Sublime, and the one who kept it neglected it. I wanted to buy it back from him, and I thought that he would sell it at a cheap price. I asked the Messenger for Allah about that and he said: Do not buy it, even if he gives it to you for a Dirham. The one who takes back his charity is like the dog that goes back to its own vomit. "'
he gave someone a horse to ride in the cause of Allah, then he saw it offered for sale and wanted to buy it. The Prophet said to him: "Do not think of buying back what you have given in charity."
'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."
That the Messenger of Allah told 'Attab bin Usaid to estimate the (harvest of) grapes, and to pay Zakah in raisins, just as the Zakah on date palms is given in died dates.