הלכה על בבא בתרא 322:5

And they, may their memory be blessed, also said (Bava Batra 167a) about this matter of contracts that a person should not write the sum of three to ten at the end of a line, lest [the one holding it] forge it and write thirty instead of three or forty instead of four or fifty instead of five (which in Hebrew only requires the addition of more letters at the end of the word), etc. And if it came out at the end of a line, we go back [and write it again] two or three times, and [then] it is possible that it will come out in the middle of the line. And they likewise said (Bava Batra 165b) that if 'one hundred' is written at the top, and 'two hundred' at the bottom - or the opposite - that it always goes according to [what is written at] the bottom. And they likewise said (Bava Batra 167b) that we write a contract for the seller or the borrower, even if the purchaser or the creditor is not with him. But we only write contracts for sharecropping or contractual work or arbitration - or any act of court - with the knowledge of them, both. And Rabbi Yitschak bar Yosef said in the chapter [entitled] Get Pashut (Bava Batra 161b) that a person must write down the existence of all erasures, entirely. And he also needs to review the subject of the contract on the last line, since we do not learn from the last line (and the review is not essential). And because of that, if the witnesses spaced their signatures one [extra blank] line below the writing, [the contract is still] fit. And that which they have become accustomed to writing 'everything is firm and valid' (Tosafot on Bava Batra 162a:1, s.v. lefi) is because the expression does not invalidate a contract and - even if we did learn from it - there is nothing [harmful] in this. And Rava said there (Bava Batra 163a) that a contract wherein he and his witnesses [write] on top of the erasure is fit. And Rav Shisha, the son of Rav Eedee said in the chapter [entitled] HaZahav (Bava Metzia 47a) that we write in the contracts, "And I acquired from him, with a vessel with which it is fitting to acquire." "With a vessel" is to exclude that of Rav Sheshet, who said that we can acquire with cows; "which it is fitting" is to exclude that of Shmuel, who said that we can acquire with a dung vessel (maroka); "to acquire" is to exclude that of Levi, who said with the vessels of the one who is making the acquisition - and Rav, and [perhaps] Rav Ashi said that that "which it is fitting" is to exclude that from which it is forbidden to derive benefit.