Q: My uncle lent me money to buy a house. When I repay the loan, may I attach a letter of profuse thanks?

A: Our Sages prohibited the lender from receiving any form of benefit from the borrower, even verbal benefit. This is referred to as ribbis devarim and is associated with the verse, “neshech kol davar” (any word). It is even prohibited for the borrower to go out of his way to greet the lender, if he was not accustomed to do so beforehand (Y.D. 160:11-12). Some authorities indicate that even simple, cordial thanks are prohibited, but many contemporary poskim allow a “Thank you” offered as common courtesy (The Laws of Ribbis, 3:14-18). Therefore, you should suffice with attaching a simple letter of thanks, without profuse writing. You may also give the lender greetings that are unrelated to the loan, such as “Mazal Tov” on the occasion of a simcha (Bris Yehuda 11(63)).

The prohibition of ribbis devarim, such as greeting the lender, is usually limited to the duration of the loan or the time of repayment, but is permitted afterwards. It is questionable, however, whether it is permitted afterwards if directly linked to the loan, such as thanking profusely for it (Bris Yehuda 10(8), but see Darchei Teshuva 160:87).