Hadith: Do not buy it back and do not take back your charity, even if he gives it to you for one dirham, for the one who takes back his gift is like the one who takes back his vomit

عن عمر بن الخطاب رضي الله عنه قال: «حَمَلْتُ على فرس في سبيل الله، فأضاعه الذي كان عنده، فأردت أن أشتريه، وظننت أنه يبيعه بِرُخْصٍ، فسألت النبي صلى الله عليه وسلم ؟ فقال: لا تَشْتَرِهِ، ولا تعد في صدقتك؛ فإن أَعْطَاكَهُ بِدِرْهَمٍ؛ فإن العَائِدَ في هِبَتِهِ كالعَائِدِ في قَيْئِهِ».
[صحيح] - [متفق عليه]

‘Umar ibn al-Khattāb (may Allah be pleased with him) reported: 'I donated a horse for Jihad. The man who had the horse neglected it, so I wanted to buy it back, thinking that he would sell it for a cheap price. I asked the Prophet about this, and he said: "Do not buy it back and do not take back your charity, even if he gives it to you for one dirham, for the one who takes back his gift is like the one who takes back his vomit."
Sahih/Authentic. - [Al-Bukhari and Muslim]

Explanation

‘Umar ibn al-Khattāb (may Allah be pleased with him) helped a man to fight in the cause of Allah by giving him a horse. However, the man did not look after the horse and did not care for it properly; he exhausted the horse until it grew weak and emaciated. So ‘Umar wanted to buy the horse back from the man, thinking that it would be cheap due to its miserable condition. As he was unsure about buying it back, he consulted the Prophet (may Allah's peace and blessings be upon him) first to clear his doubts. The Prophet (may Allah's peace and blessings be upon him) forbade him from buying it back, even for a meager price, as he had given the horse for Allah's sake. One should not follow up on donations or feel attached to them. The person who received the donation may feel obliged to sell it back to the donor for a low price. In this case, the donor will receive back part of his charity at no price. Also, when charity is given, it takes with it one's bad deeds, so one should not take them back and should not look back. Buying back one's charity is like retracting one's charity, despite the price paid.