Buying a foreclosed house if it is sold at auction because the borrower is not able to pay off the riba-based loan
Question

                Somebody mortgaged his house in return for a riba-based loan, and at the end of the agreed period, because the home owner was unable to pay off his debt, the riba-based institution sold the house through the court at a public auction. Because the house is opposite to a mosque, some good brothers thought to buy it from the one who bought it at the auction and add its area to the mosque. 

My question is: 

1. What is the ruling on mortgaging the house in return for the riba-based loan?

2. What is the ruling on buying this house that was sold without the consent of its owner who was not able to pay off the riba-based loan?

3. Does this purchase come under the heading of buying something that was unlawfully confiscated?

4. What is the ruling on buying this house from the second purchaser, when the story of its sale is known, and adding it to the area of the mosque?.
Praise be to Allah.
      Firstly: 

    Borrowing money on the basis of riba is emphatically 
    forbidden and is a major sin, because Allah, may He be glorified and 
    exalted, says (interpretation of the meaning):

    “O you who believe! Fear Allaah and give up what remains 
    (due to you) from Ribaa (from now onward) if you are (really) believers.

    279. And if you do not do it, then take a notice of war 
    from Allaah and His Messenger but if you repent, you shall have your capital 
    sums. Deal not unjustly (by asking more than your capital sums), and you 
    shall not be dealt with unjustly (by receiving less than your capital sums)”

    [al-Baqarah 2:278-279]

    And Muslim (1598) narrated that Jaabir (may Allah be pleased 
    with him) said: The Messenger of Allaah (peace and blessings of Allaah be 
    upon him) cursed the one who consumes riba and the one who pays it, the one 
    who writes it down and the two who witness it, and he said: they are all the 
    same. 

    So it is not permissible to take out a riba-based loan or to 
    mortgage the house in order to take out such a loan. 

    Secondly: 

    The one who takes out a riba-based loan is not obliged to pay 
    the interest; rather he has to pay back the original amount of the loan 
    only. If he is not able to do that, and he gave something to the lender as 
    collateral, it is permissible to sell the collateral in two cases: 

    1.If the borrower gave permission 
    to sell it, whether he gave permission at the time the contract was drawn up 
    or at the time payment became due.

    2.If the court rules that it is 
    to be sold.

    It says in Zaad al-Mustaqni‘: When the debt is due to 
    be repaid and he refuses to pay, if the borrower gave permission to the 
    lender to sell it, he may sell it and pay off the debt, otherwise the judge 
    may force him to pay off the debt or sell the collateral. If he does not do 
    that, the judge may sell it to pay off the debt. End quote. 

    The court may appoint someone to sell it, whether it appoints 
    the lender or someone else. 

    It is stipulated that the one who sells it, whether it is the 
    lender or someone else appointed by the judge, should sell it for the market 
    price and not for less than that. 

    It says in Mughni al-Muhtaaj, 3/71: 

    The collateral should not be sold except for the current 
    price in the local currency of something similar; if that condition is not 
    met, then the sale is not valid. But if it is a little below the going 
    price, which often happens when people bargain, that does not matter because 
    they usually overlook that. End quote. 

    Based on that, if the item is sold because the borrower is 
    unable to pay off the original loan, and that is done on the orders of the 
    court and it is sold by public auction, there is nothing wrong with that and 
    it does not matter if the seller did not give consent, because selling an 
    item when its owner is reluctant is valid if he is forced to do so for a 
    legitimate reason, and this is not regarded as buying something that was 
    unlawfully confiscated or taken by force. 

    Shaykh Ibn ‘Uthaymeen (may Allah have mercy on him) said: The 
    words ‘so it [the transaction] is not valid if he is forced to sell it 
    without a legitimate reason’ – he commented on this by saying: If it is sold 
    when the owner is reluctant for a legitimate reason, there is nothing wrong 
    with that, because this is restoring the dues of others, i.e., if we force 
    someone to sell something for a legitimate reason, this is restoring the 
    dues of others, and is not wrongdoing or transgression. 

    For example: a person gave his house as collateral to someone 
    for a debt that he owed to him. The time for repayment came and the lender 
    asked for his money, but the borrower who owed the debt refused to pay. In 
    this case, the borrower may be forced to sell his house, so that he can pay 
    off what he owes to the lender.

    Another example: A piece of land was owned by two people, and 
    it was a small piece of land that could not be divided. One of the partners 
    asked the other to sell, but the other partner refused. In this case the 
    land may be sold despite the objections of the one who refused, because 
    there is a legitimate reason, which is to ward off harm from his partner. 

    So the guideline is: If forcing someone to sell is for a 
    legitimate reason, the sale is valid even if the seller does not give 
    consent to it. Because in this case we are not committing a sin by wronging 
    him or by any other means, so it is permissible. End quote from al-Sharh 
    al-Mumti‘, 8/108 

    Thirdly: 

    There is nothing wrong with buying a house from the second 
    purchaser and adding it to the mosque. 

    And Allah knows best.