Reference for Arakhin 39:37

מלוה הכתובה בתורה ככתובה בשטר דמיא

But here he was really mistaken, in that he believed that just as there is valuation to 'my hand or liver', there is one to 'my foot or hand', but he never meant the market-value; therefore he informs us [that he must pay the market-value nevertheless].

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