For the source text click/tap here: Bava Metzia 63
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The Gemara teaches that according to Rebbi Yehudah, "Tzad Echad b'Ribis" is permitted. "Tzad Echad b'Ribis" refers to a transaction which can have two possible outcomes, one of which involves Ribis.
He maintains that a transaction that involves Ribis is prohibited only when Ribis definitely will be involved, but not when the presence of Ribis depends on the outcome of the transaction. The classic case of "Tzad Echad b'Ribis" is where a borrower gives his field to the lender and stipulates that if he does not repay the loan, the lender will acquire the field (for the money of the loan) retroactively.
In the interim, the lender eats the produce of the field. According to the Tana Kama in the Beraisa, this is forbidden as Ribis, because if the borrower pays back the loan on time and repossesses his field, the sale will be invalidated and all the fruit that the lender ate will have constituted Ribis.
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