‘Umar b. al-Rhattab said

I provided a man with a horse to ride in God’s path, but as the one who had it did not look after it well, I wanted to buy it and thought he would sell it at a cheap price. I therefore asked the Prophet, but he said, “Do not buy it, and do not take back what you gave as sadaqa even if he gives it to you for a dirham, for the one who takes back what he gave as sadaqa is like a dog which returns to its vomit.” A version has, “Do not take back what you gave as sadaqa, for the one who does so is like one who takes back what he has vomited.” (Bukhari and Muslim.)