The 237th prohibition is that we are forbidden from being involved between the borrower and the lender in a loan with interest. We cannot act as guarantor for either one of them, testify for them, or write a document for them regarding their agreement involving interest.
The source of this prohibition is G‑d's statement (exalted be He), "Do not place interest upon him." In the words of tractate Bava Metzia: "The guarantor and witnesses only transgress the prohibition, 'Do not place interest upon him.' " There it is explained that the scribe has the same status as the guarantor and witnesses.
It is also explained there that this prohibition, "Do not place interest upon him" — although it refers to the intermediaries — also includes the lender. Therefore, one who lends money with interest transgresses six prohibitions:
1) "Do not press him for repayment."
2) "Do not make him pay interest (neshech) for your money."
3) "Do not make him pay interest (marbis) for your food."
4) "Do not take interest from him."
5) "Do not place interest upon him."
6) "Do not place a stumbling block before the blind."
Our Sages said there that: "The following transgress prohibitions: the lender, the borrower, the guarantor, and the witnesses. The Sages said: 'The scribe too.' They transgress 'do not make him pay,' 'do not take,' 'do not press him,' 'do not place,' and 'Do not place a stumbling block before the blind.' " Abaye said in the Gemara, "The lender transgresses all of them; the borrower — 'do not deduct' and 'do not place a stumbling block before the blind'; the guarantor and witnesses — 'do not place interest upon him.' "
When one transgresses this prohibition — if it is ribis k'tzutza (interest by Torah law) — the interest is taken away and given back to the one who paid it.
