When paying back a loan, is saying “thank you” ribbis?
No, saying “thank you” is not considered ribbis.
The Gemara and the Shulchan Aruch write that if a person was not used, prior to the loan, to greeting his creditor, and he begins to greet him after the loan, this is considered ribbis devarim, “ribbis of words.”
The Iggros Moshe (Yoreh De’ah 1:80) writes that on account of this principle, it would be a problem to say “thank you” for a loan.
However, the Minchas Shlomo (1:27), after explaining that the blessing of “tizkeh le-mitzvos” (you should have the merit of performing mitzvos) appears to be problematic, writes that there is no prohibition is saying “thank you,” because the concept of “thank you” is only that a person does not wish to be ungrateful to any person, and a basic expression of gratitude does not fall under the category of ribbis devarim.
He explains that this is not similar to the Gemara concerning greeting somebody, because greeting somebody (with Shalom) is a blessing, and it is forbidden to give a blessing on account of the loan. This is not similar to a simple expression of gratitude, which is permitted.
The sefer “Ribbis Le-Or Ha-Halachah” cited from Rabbi Ben-Zion Abba Shaul that one can be lenient in this matter, and this is also the ruling given by Rabbi Elchanan Peretz in his Neshech Kesef (Chap. 2).