If Reuven is lends money to Shimon’s trust account but not directly to him and Reuven is the trustee on the account anyways so that he’ll take back the money he lends to it, at a later time, can Shimon say thank you to Reuven since he’ll get the money from his own trust account and it’s his trust account that is paying Reuven back, not him?
Thank you for your question.
My understanding of you question is that you loaned money to his trust account, and the money is intended for Shimon to use, but instead of putting into his personal bank account you put it into the “trust” account (which is really him, except that now he has a legal loophole for taxes etc., and that the assets are not “technically his.) The fact that you can take the money back when you like does not diminish the fact that it is a loan. In general though, there is controversy if one may say thank you, after getting a loan. Although this idea is mentioned in the poskim, however a number of poskim say that nowadays it is the normal way of talking, and it isn’t being said out of thanks and payment for the loan rather out of derech eretz, therefore it is permitted.
Shulchan Aruch Horav Hilchos Ribbis-9, Chut Shani- Ribbis pg. 63. See Minchas Shlomo 1-27(1) that R’ S.Z. says that it is permitted however in the footnote it says that he retracted this ruling because of the Shulchan Aruch Horav. However, see Nesivos Shalom pg. 107, that R’ S. Z. used to say often say that saying Thank you is a matter of manners and derech eretz and not an extra payment for the loan. He also bring this from R’ Eliyashiv zt”l.