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." (Using translation from Tirmidhī 668)
أَنَّ عُمَرَ حَمَلَ عَلَى فَرَسٍ لَهُ فِي سَبِيلِ اللهِ ثُمَّ رَآهَا تُبَاعُ فَأَرَادَ أَنْ يَشْتَرِيَهَا فَقَالَ لَهُ رَسُولُ اللهِ ﷺ لَا تَعُدْ فِي صَدَقَتِكَ