It is narrated from Sayyiduna Umar bin Khattab (may Allah be pleased with him) that I provided a horse for jihad, but the one who had it wasted it. I intended to buy it from him, thinking (since the horse had become weak) he would sell it cheaply. So I asked the Messenger of Allah (peace and blessings be upon him) about this. He (peace and blessings be upon him) said: "Even if he gives it to you for one dirham, do not buy it, and do not take back your charity. The one who takes back his charity is like a dog that vomits and then licks it up."
Hadith Referenceالسنن المأثورة / باب ما جاء في صدقة الفطر / 372
Hadith Takhrijصحیح بخاری، الزکاۃ، باب ہل یشتری صدقتہ، رقم : 1490، صحیح مسلم، الہبات باب کراہۃ شراء الانسان ...... الخ، رقم : 1620