Umar, may Allah be pleased with him, donated a horse to be used in the cause of Allah. Umar gave it to the Messenger of Allah ﷺ so that he could give it to a man to ride. Later, Umar was informed that the man had put the horse up for sale. So Umar asked the Prophet ﷺ whether he could buy it back.The Prophet ﷺ replied, "Do not buy it, and do not take back your charity."
حدثنا يحيى عن عبيد الله أخبرني نافع
أن عمر رضي الله تعالى عنه حمل على فرس فأعطاها عمر رسول الله ﷺ ليحمل عليها رجلا فأخبر عمر أنه قد وقفها يبيعها قال فسأل عن ذلك النبي ﷺ يبتاعها قال لا تبتعها ولا تعد في صدقتك