Should he sell lend to one who he knows will use it as collateral for a riba-based loan?
Question

                What is the ruling on selling a piece of land to a person who wants to use it as collateral to obtain a riba-based loan?.
Praise be to Allah.If it is known that this 
    person is buying the land in order to use it as collateral to get a 
    riba-based loan, then it is not permissible to sell it to him, so that you 
    will not be helping him in that sin. Allaah says (interpretation of the 
    meaning): 
“Help you one another in 
    Al‑Birr and At‑Taqwa (virtue, righteousness and piety); but do not help one 
    another in sin and transgression. And fear Allaah. Verily, Allaah is Severe 
    in punishment”
[al-Maa’idah 5:2]
The fuqaha’ have stated 
    that it is haraam to sell things that may be used to help in committing sin, 
    such as selling grapes to one who will make wine from them, or selling 
    weapons at times of turmoil, and so on. 
Ibn Qudaamah (may Allaah 
    have mercy on him) said: To sum up: selling juice to one who it is believed 
    will use it to make wine is haraam. 
Then he said: The same 
    ruling applies to everything that is intended for haraam purposes, such as 
    selling weapons to those who are waging war (against the Muslims) or to 
    bandits, or at times of turmoil, or selling a slave woman to be a singer or 
    to be hired out, or renting his house for alcohol to be sold in it or to be 
    used as a church, and so on. This is haraam and the rental contract is 
    invalid. Ibn ‘Aqeel said: Ahmad (may Allaah have mercy on him) stated 
    several issues in which he drew attention to that, and he said concerning 
    butchers and bakers: If he knows that the one who is buying from him is 
    going to invite people to drink alcohol with the food, then he should not 
    sell to him. End quote from al-Mughni (4/154). 
Shaykh al-Islam Ibn 
    Taymiyah (may Allaah have mercy on him) said: It is not right to sell to one 
    who intends to do haraam with (the purchased items), such as juice which 
    will be used to make wine, if he knows that is the case, as is the view of 
    Ahmad and others, or if he thinks that this is likely, which is one of the 
    two views. This is supported by the view of his companions, who said: If the 
    landlord thinks that the renter is renting the house for sinful purposes, 
    such as selling alcohol and the like, it is not permissible for him to rent 
    that house out to him, and the rent contract is not valid. Selling and 
    renting are the same. End quote from al-Fataawa al-Kubra (5/388). 
And he said in Sharh 
    al-‘Umdah (4/386): Every garment that he thinks will most likely be used 
    for sinful purposes, it is not permissible to sell it to or sew it for one 
    who will use it for sinful and wrong purposes. End quote. 
And Allaah knows best.