As we learned on yesterday’s daf someone with a personal, vested interest in a disputed property will not be permitted to testify about that case. Our Gemara examines what might be considered a personal interest in property.
Ravin bar Shmuel quotes Shmuel as saying that if someone sells a field to his friend, and stipulates that he does not take responsibility for the sale (she-lo be-aḥarayut, i.e. should the land be taken from him for any reason, he will not return the purchase price), if someone else comes and claims the land, the seller cannot testify on behalf of the purchaser. This is true even though according to the sale agreement he will not be responsible to refund the money that he got paid. The reason for this is because he benefits from having the field in the hands of the person he sold it to, since if he has debts, in theory they can be collected from that field, which has a lien on it guaranteeing the money that he owes.
This is only true if he sold a house or a field to another. If he sold a movable object, like cattle or an article of clothing, then he would be allowed to testify in support of the purchaser, since a debt cannot be collected from a movable object that has been sold. Therefore he has no personal interest in keeping the object in the hands of the buyer and his testimony will be accepted.
The reason that real estate will guarantee a debt even after it is sold, but movable objects will not is because of what the Gemara calls a kol – literally a sound or a voice. When real estate is sold, word of the sale soon becomes public knowledge and the purchaser should be aware that he is taking a risk when he buys this property. When movable objects are sold, however, it has no kol, so we cannot anticipate that a purchaser will realize that the object may have a lien on it.