Expressing Gratitude

Even a borrower who blesses someone who lends him money is in clear violation of rabbinic interest.
It is very normal for people to borrow money to cover the expense of printing a sefer. Naturally, the author wishes to give some kind of thanks or bless the lender in his book, since without his generosity there would be no publication.
Yet the Erech Shai, zt”l, prohibits this. Rav Moshe Feinstein, zt”l, suggests a simple way around this prohibition. One can certainly write that “the lender will be blessed from heaven.” The reason this is permitted is because the borrower is not blessing the lender, he is merely writing what will be as a result of his generosity.
Rav Shlomo Zalman Auerbach, zt”l, pointed out that although people know that one may not thank another, they say, “Tizku l’mitzvos.” He continued, “This is not a halachically permitted alternative, since Tosefos in Kiddushin 8 says clearly that tizku l’mitzvos is rivis. It seems to me that they would be better off saying thank you, since this in not a blessing and may be different from giving a brachah which is clearly prohibited.”
But when someone showed him that the Shulchan Aruch HaRav Baal HaTanya explicitly prohibits saying “thank you,” Rav Shlomo Zalman backtracked. “I thought that the rabbis only prohibited giving a blessing or praising the lender since he has pleasure from this. But thank you is no brachah at all, and how can one take a loan and act as though the lender did him no kindness? This is surely a contradiction to derech eretz and is presumably prohibited. But when I was shown the Shulchan Aruch HaRav I changed my mind.”