Once I gave a horse in Allah's Cause (in charity) but that person did not take care of it. I intended to buy it, as I thought he would sell it at a low price. So, I asked the Prophet ﷺ about it. He said, "Neither buy, nor take back your alms which you have given, even if the seller were willing to sell it for one Dirham, for he who takes back his alms is like the one who swallows his own vomit."  

سَمِعْتُ عُمَرَ ؓ يَقُولُ حَمَلْتُ عَلَى فَرَسٍ فِي سَبِيلِ اللَّهِ فَأَضَاعَهُ الَّذِي كَانَ عِنْدَهُ فَأَرَدْتُ أَنْ أَشْتَرِيَهُ وَظَنَنْتُ أَنَّهُ يَبِيعُهُ بِرُخْصٍ فَسَأَلْتُ النَّبِيَّ ﷺ فَقَالَ لاَ تَشْتَرِ وَلاَ تَعُدْ فِي صَدَقَتِكَ وَإِنْ أَعْطَاكَهُ بِدِرْهَمٍ فَإِنَّ الْعَائِدَ فِي صَدَقَتِهِ كَالْعَائِدِ فِي قَيْئِهِ  

I gave a horse to be used in Allah's Cause, but later on I saw it being sold. I asked the Prophet ﷺ whether I could buy it. He said, "Don't buy it and don't take back your gift of charity."  

عُمَرُ بْنُ الْخَطَّابِ ؓ حَمَلْتُ عَلَى فَرَسٍ فِي سَبِيلِ اللَّهِ فَرَأَيْتُهُ يُبَاعُ فَسَأَلْتُ النَّبِيَّ ﷺ آشْتَرِيهِ فَقَالَ لاَ تَشْتَرِهِ وَلاَ تَعُدْ فِي صَدَقَتِكَ