ʿUmar gave a horse to be used in Allah's Cause, but later on he found it being sold. So, he intended to buy it and asked 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