http://revach.net/article.php?id=3825

Section: Halacha   Category: Tshuvos
Rav Moshe Feinstein: Thanking For A Loan In The Dedication Of A Sefer

It is forbidden for a borrower to say thank you to the lender because this is considered lending with interest.  Ribbis Devarim is forbidden even though it is only words and not a monetary.  Based on this Rav Moshe Feinstein was asked (YD1:80), if someone borrowed money to publish a Sefer, may the Michaber thank the lender in the Sefer for making the publication possible?

Rav Moshe says that he may not do so since this constitutes Ribbis Devarim.  However, says Rav Moshe, you may write that the benefactor will be blessed from Shamayim for his act of kindness in lending money for the publication.  This is not a thank you, it is merely  a notification of the fact that the lender did a good deed and Hashem will pay him back.

Even though the lender enjoys the publicity it is still permissible, since Chazal say that it is a mitzva to publicize those who do mitzvos, unless they do not want the publicity.

Important Note: We try to convey the Tshuva to the best of our ability. We admit that our understanding may not be accurate. One should learn the tshuva to verify the accuracy of our interpretation.  Please also understand that this Tshuva may not be the final word on this topic. One should consult a Rav before drawing any conclusions.