The Father (not frum) of a Baal Teshuva asked his son for a loan. The son agreed to give his Father the loan on condition that he starts to keep Shabbos. Is this a problem of ribis?
Answer:
No, this does not involve ribbis, because keeping Shabbos is in any case obligatory, and is something between a person and Hashem.
Poskim write that giving a loan on condition that the borrower will fulfill his already present obligation is not considered ribbis.
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Best wishes.