Ibn 'Umar reported that 'Umar b. al-Khattib (Allah be pleased with him) donated a horse in the path of Allah and (later on) he found it being sold, and he decided to buy that. He asked the Messenger of Allah (ﷺ) about it. whereupon he (the Holy prophet) said:Don't buy that and do not get back what you gave in charity.
حَدَّثَنَا يَحْيَى بْنُ يَحْيَى، قَالَ قَرَأْتُ عَلَى مَالِكٍ عَنْ نَافِعٍ، عَنِ ابْنِ عُمَرَ، أَنَّ عُمَرَ، بْنَ الْخَطَّابِ حَمَلَ عَلَى فَرَسٍ فِي سَبِيلِ اللَّهِ فَوَجَدَهُ يُبَاعُ فَأَرَادَ أَنْ يَبْتَاعَهُ فَسَأَلَ رَسُولَ اللَّهِ صلى الله عليه وسلم عَنْ ذَلِكَ فَقَالَ " لاَ تَبْتَعْهُ وَلاَ تَعُدْ فِي صَدَقَتِكَ " .
Muslim
The Book of Gifts • Hadith 5