Is the seller's agreement to sign usurious loan documents for the buyer, as a condition for the buyer to obtain the loan and complete the sale, considered a sin for the seller?

Allah has forbidden usury and made it one of the grave sins. The Messenger of Allah, peace and blessings be upon him, cursed the one who consumes usury, the one who feeds it, its scribe, and its two witnesses, and said: "They are all equal." This indicates the prohibition of assisting in usury.

So, if a person is going to take out a usurious loan to buy a house, and the approval depends on your signature, then it is forbidden for you to sign; because that is assisting in a usurious transaction, due to the saying of Allah the Almighty: (And do not assist each other in sin and aggression).

However, if the person takes out a usurious loan without your involvement, and then brings that money to buy the house from you, there is no harm upon you in selling the house to him, for transacting with someone who has borrowed with usury, whether buying or selling, is permissible as long as the transaction itself is permissible.