Reference for Bava Metzia 154:8
רשב"ג אומר מלמדין אותן שלא יחזרו כיצד כותב לו אני פלוני בן פלוני מכרתי שדה פלונית לפלוני באלף זוז ונתן לי מהם מאתים זוז והריני נושה בו ח' מאות זוז קנה ומחזיר לו את השאר אפי' לאחר כמה שנים
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>, upon which he has paid me two hundred <i>zuz</i>, and now I am his creditor for eight hundred <i>zuz</i>.' Thus he [the vendee] acquires the title thereto, and must repay him the rest, even after many years.<span class="x" onmousemove="('comment',' The point is that the other 800 zuz are described on this bond not as the balance due but as an ordinary debt, and therefore does not affect the ownership of the field, which passes to the buyer on payment of money. ');"><sup>7</sup></span>