Umar gave a horse to be used in the cause of Allah. Then he saw it being sold, so he wanted to buy it, but the Prophet said: "Do not take back what you have given in charity."  

أَنَّهُ حَمَلَ عَلَى فَرَسٍ فِي سَبِيلِ اللَّهِ ثُمَّ رَآهَا تُبَاعُ فَأَرَادَ أَنْ يَشْتَرِيَهَا فَقَالَ النَّبِيُّ ﷺ لاَ تَعُدْ فِي صَدَقَتِكَ