The lender, borrower, guarantor and witnesses violate the prohibitions against ribbis (Y.D.160:1). Therefore, a lawyer drafting a document or a notary must be vigilant and refrain from servicing a ribbis agreement (see Shach, Y.D. 160:1). Included in this group is someone who brokers a loan between a Jewish lender and Jewish borrower (Shach, ibid. 1).

However, it is permitted to work as a mortgage broker for a non-Jewish lender since the broker is neither the lender nor a guarantor. This is permitted when it is clear that the broker is not lending his own funds (for examples see Shach 168:71 and Bris Yehudah 31:[5]). If it is not clear, it is prohibited due to mar’is ayin (the appearance of impropriety [Y.D. 168:23]).

Whether the buyer may use the property for free depends on the situation. If the seller does not have access to the earnest money, it is permitted. Since the buyer did not lend any money to the seller, there is no prohibition of ribbis (Minchas Yitzchak 5:19).

However, there are times when the seller wants immediate access to the earnest money, e.g., to purchase his new home. If the buyer permits the seller to use those funds before the closing, he is essentially loaning him the money. Consequently, if the buyer uses the property in consideration of the loan, they have encroached on the prohibition of ribbis (Y.D. 174:6).

Even though the seller cannot lease the property between the time that it goes under contract and the closing and it falls into the category of zeh neheneh v’zeh lo chaser — one party benefits and the other party does not lose, nevertheless, if they have a lender/ borrower relationship, it is prohibited. Certainly in your case, if the seller allows you to use the property without paying rent, it may constitute a Biblical violation of ribbis (Y.D. 166:2; Shach 6; and see Chavos Daas 3).

Once the price was set, the seller may not offer a discount if the buyer prepays some of the money before the closing. The rationale is that since the buyer is not obligated to pay until the closing, prepayment or allowing the seller to use the earnest money constitutes a loan, and as a lender he may not receive any benefits for issuing the loan (Y.D. 173:7). Therefore, since these arrangements involve ribbis, a Heter Iska is necessary.

Alternatively, it is permitted if the buyer acquires a part of the property commensurate with the money that he prepaid.

When someone loans a significant amount of money to be used to purchase a specific property. and stipulates that if the buyer decides to sell the property, he has the right of first refusal.

This is sometimes a ribbis concern and should be discussed with a Rav.