Bava Batra 167A

ומשוי להו זוזי; מאי אמרת ⁠– שית מאה איסתירי וזוזא, שית מאה זוזי וחד זוזא? יד בעל השטר על התחתונה.

and change them into a smaller number of dinars. Therefore, what can you say? The highest and lowest remaining possibilities are: 600 istira and a dinar, and 600 dinars and one more dinar. The guiding principle is that the holder of the document is at a disadvantage, and the lesser of these two values is assumed.

אמר אביי: האי מאן דבעי למחוי חתימות ידיה בבי דינא, לא לחוי בסוף מגילתא; דלמא משכח לה אחר וכתיב דמסיק ביה זוזי, ותנן: הוציא עליו כתב ידו שהוא חייב לו ⁠– גובה מנכסים בני חורין.

§ Abaye said: With regard to this one who needs to show his signature in court for the purpose of corroborating his signature on a document, he should not show it by writing it at the end of the parchment, lest another, unscrupulous, person find the parchment and write above the signature that the signatory owes him money. And such a document would be valid, as we learned in a Mishnah (175b): If one presents to a debtor a document in the handwriting of the debtor stating that he owes money to him, but without witnesses signed on the document, the creditor can collect only from unsold property, i.e., property that is currently in the possession of the debtor.

ההוא בזבינא דאתא לקמיה דאביי, אמר ליה: ניחזי לי מר חתימות ידיה, דכי אתו רבנן מחוו לי, מעברנא להו בלא מכסא. אחוי ליה בריש מגילתא. הוה קא נגיד ביה, אמר ליה: כבר קדמוך רבנן.

The Talmud relates: There was a certain Jewish tax collector who came before Abaye and said to him: Let the Master show me his signature on a piece of paper to keep in my records, as when rabbis come to me and show me a note with your signature on it, attesting to the fact that they are Torah scholars, I let them pass without paying the tax. Abaye showed him his signature at the top of the parchment, though the unscrupulous tax collector kept pulling the parchment away from Abaye so that the signature would be at the bottom. Abaye noticed this and said to him: The rabbis have already anticipated people such as you and advised that one should never write his signature at the bottom of a paper.