Why would the Rabbanan who argue with Reb Meir potentially agree that a lein on something that has not been purchased yet could be valid?
Zev, Seattle, Washington
The Rashbam (DH Ela) gives two explanations. Firstly because the property which is to be Meshubad already exists, whereas the in the case of Peiros Dekel, the Rabanan say that the Kinyan cannot take place because the article does not exist at all. Alternatively, the Rashbam explains that the Rabanan instituted that the Shibud should take effect to protect the borrower.