The Darkei Teshuvah (160:81) quotes the Marbeh Torah (s.k. 29) who writes that it is forbidden to lend money to a talmud chocham on condition that he davens for a refuah sheleimah for the lender, as it is like ribbis devorim that was fixed at the time of the loan, and is a דררא של איסור דאורייתא – very close to being an issur de’O’raisa. He adds: We can’t say there is a mitzvah for the talmud chocham to daven for the sick person, as the mitzvah to daven is upon the ill person himself who is in dire situation, and not the talmud chocham.
Although the Rema in Hilchos Bikur Cholim (Yoreh Deah 335:10) writes: “Some say, if one has a sick person in his house, he should go to the chocham [sage] of the city and ask the chocham to plead for mercy on behalf the ill person”, which seems to imply that there is a mitzvah to go to ask the chocham to daven for the sick person, when the issur of ribbis is being violated then it’s a mitzvah haboh b’aveirah [mitzvah common through an act of sin]. Although when there is a situation of pikuach nefesh [danger to life] we are more lenient in terms of halachah, that’s only when the refuah [heeling method] is natural, if, however, it’s an inyan of segulah, we aren’t, therefore, there is no reason to be lenient on the prohibition of ribbis devorim when it comes to davening for someone who is sick.
However, he concludes: The borrower is certainly obligated to daven for the sick person, as gemilas chasodim [acts of kindness, such as lending an interest free loan] is greater than tzedokah, and if when one receives tzedokah he should daven for the contributor, certainly when one receives gemilas chasodim he should daven for the contributor. Therefore, the borrower may say to the sick lender, “lend me money, and I will daven for you”, however, he shouldn’t stipulate “lend me on condition, I will daven for you”. He should miss out the stipulation.
