ʿUmar bin Al-Khattab gave a horse to be ridden in Allah's Cause and then he found it being sold. He intended to purchase it. So, he consulted Messenger of Allah ﷺ who said, "Don't buy it and don't take back your gift of charity."  

أَنْ يَبْتَاعَهُ فَسَأَلَ رَسُولَ اللَّهِ ﷺ فَقَالَ لاَ تَبْتَعْهُ وَلاَ تَعُدْ فِي صَدَقَتِكَ  

I heard ʿUmar bin Al-Khattab saying, "I gave a horse to be ridden in Allah's Cause and the person who got it intended to sell it or neglected it. So, I wanted to buy it as I thought he would sell it cheap. I consulted the Prophet ﷺ who said, "Do not buy it even if for one Dirham, because he who takes back his gift is like a dog swallowing its vomit."  

أَنَّهُ بَائِعُهُ بِرُخْصٍ فَسَأَلْتُ النَّبِيَّ ﷺ فَقَالَ لاَ تَشْتَرِهِ وَإِنْ بِدِرْهَمٍ فَإِنَّ الْعَائِدَ فِي هِبَتِهِ كَالْكَلْبِ يَعُودُ فِي قَيْئِهِ