When someone borrows money and the loan is formalized in a shetar – a promissory note – it was commonplace to include ahrayut nekhasim – a guarantee that the loan would be paid from real estate held by the borrower at the time of the loan. If the borrower sold his land to a third party before the loan was paid off, the land is sold with a lien on it and in the event that the borrower cannot pay his debt when it comes due, the lender will be able to collect what is owed to him from the purchaser.
Our Gemara examines a position put forward by Shmuel who rules that if the borrower cannot pay, then the lender can collect not only from the guaranteed field itself, but also from any increase in the value of the field that derives from the investment that the third-party purchaser made in the field. Rava explains that this rule is based on the standard contract that was written at the time that a field was sold, where the seller guarantees to the purchaser that he will make sure that the purchaser will be fully reimbursed should there be any problem with the purchase, the investment or the profits stemming from the sale. The witnesses on this contract then sign a statement that the seller agreed to all the stipulations that it contains – Rabbeinu Hananel and the Rambam suggest that the witnesses’ signatures indicate that the purchaser agrees to all of the stipulations, indicating that he recognizes that his profits and investment may be taken, but is guaranteed reimbursement for them.
According to the Rosh, by pointing out the language of the contract, Rava is bringing support to Shmuel’s ruling that the lender can collect from the purchaser’s investment, but also gives the reason for it. If the purchaser would lose his investment, the Sages would not have established such a rule. Since he is guaranteed reimbursement, the Sages preferred to establish the law in favor of the lender – she-lo tinol delet bi-fnei lovin – in order to encourage people to lend money (literally “so that the door should not be locked in the face of borrowers”).