Reference for Bava Metzia 154:16

לא קשיא הא דקא עייל ונפיק אזוזי הא דלא קא עייל ונפיק אזוזי

'R. Simeon b. Gamaliel said: They are instructed [so to act as] to make it impossible [for either] to withdraw. How so? He [the vendor] must draw up a deed, stating. "I [so-and-so have sold such and such a field to so-and-so for a thousand <i>zuz</i>, etc."' Hence, it is only because he writes thus;<span class="x" onmousemove="('comment',' I.e., describing the balance as an ordinary debt. ');">¹⁴</span>

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