I was just looking over the Talmud in Bava Metzia and I saw an important point. On page 104.The Mishna brings two conditions in which in renting there is a meaning that is implied "this field" in the case where one says, "I want to rent this apartment from you." These are the two strongest conditions: "This" and the person that want to rent the place. But you don't know that the  Mishna required both conditions. All the Mishna does is juxtapose this with the case of a serf. With the serf there is no requirement that the field remain the same. [It is entirely conceivable that the mishna requires neither condition and only make the difference to depend on whether the case is serfdom or rent. Or that it requires both of the strongest conditions. Or anywhere in the middle also for that matter]
It is hard to know what the Mishna means.

So when we get to Ravina and Shmuel things pick up. Ravina says, all that matters is the "this," and Shmuel says all that matters is who said it (the person that wants to rent, or the one that wants to rent out his place).
Here is where the Rambam comes in and brings the Tosepfta (teaching) that says exactly the same things as the Mishna in the case of renting and says openly it does not matter if the word "this" was used. This is a proof to the Rambam that  if the renter said it, then it does not matter. My point is it is hard to see why the Tospefta should be assumed to be a proof of the meaning of the Mishna.