He bought three pieces of land then he sold two of them
Question

                There are three pieces of land, each of which is being sold for 20,000 riyals, or all three for 45,000 riyals. Is it permissible for me to buy all three pieces of land if I know that there are two people who will buy two of the pieces of land for 20,000 riyals each, and I will keep the third, and I know these two buyers?.
Praise be to Allah.It is permissible for you 
    to buy these pieces of land for 45,000 riyals then sell two of them for 
    40,000 and keep the third piece for yourself. 
‘Urwah ibn al-Ja’d (may 
    Allaah be pleased with him) bought two sheep for a dinar, then he sold one 
    of them for a dinar, then he went back to the Prophet  (peace and 
    blessings of Allaah be upon him) with one dinar and one sheep, and the 
    Prophet  (peace and blessings of Allaah be upon him) prayed for blessing 
    for him. Narrated by al-Bukhaari (3643). 
What matters is that your 
    purchase of the land be a permissible purchase, with no wrongdoing or 
    transgression against anyone. 
Do not sell any of them 
    until you have taken full possession of them, because of the report narrated 
    by Abu Dawood (3503), al-Tirmidhi (1232) and al-Nasaa’i (4613) from Hakeem 
    ibn Hizaam (may Allaah be pleased with him) who said: I asked the Prophet 
     (peace and blessings of Allaah be upon him): O Messenger of Allaah, a 
    man comes to me and asks me to sell him something that I do not have to sell 
    to him, but I go to the marketplace and buy it for him. He said: “Do not 
    sell what you do not have.” Classed as saheeh by al-Albaani in Saheeh 
    al-Nasaa’i. 
It is not permissible for 
    you to sell the land to these two men before you have bought it, so buy it 
    first, then sell it to them if you wish. 
And Allaah knows best.