You should not buy it

Bukhari :: Book 4 :: Volume 51 :: Hadith 31

Narrated Ibn Abbas:

A man said to Allah's Apostle , "My mother died, will it benefit her if I give in charity on her behalf?" The Prophet replied in the affirmative. The man said, "I have a garden and I make you a witness that I give it in charity on her behalf."

Bukhari :: Book 4 :: Volume 51 :: Hadith 32

Narrated Anas:

When the Prophet ordered that the mosque be built, he said, "O Bani An-Najjar! Suggest to me a price for this garden of yours." They replied, "By Allah! We will demand its price from none but Allah."

Bukhari :: Book 4 :: Volume 51 :: Hadith 33

Narrated Ibn 'Umar:

When 'Umar got a piece of land in Khaibar, he came to the Prophet saying, "I have got a piece of land, better than which I have never got. So what do you advise me regarding it?" The Prophet said, "If you wish you can keep it as an endowment to be used for charitable purposes." So, 'Umar gave the land in charity (i.e. as an endowments on the condition that the land would neither be sold nor given as a present, nor bequeathed, (and its yield) would be used for the poor, the kinsmen, the emancipation of slaves, Jihad, and for guests and travelers; and its administrator could eat in a reasonable just manner, and he also could feed his friends without intending to be wealthy by its means."

Bukhari :: Book 4 :: Volume 51 :: Hadith 36

Narrated Ibn 'Umar:

Once 'Umar gave a horse in charity to be used in holy fighting. It had been given to him by Allah's Apostle . 'Umar gave it to another man to ride. Then 'Umar was informed that the man put the horse for sale, so he asked Allah's Apostle whether he could buy it. Allah's Apostle replied, "You should not buy it, for you should not take back what you have given in charity."