QUESTION: The Gemara rules that when a Yisrael rents a cow from a Kohen or a Kohen rents a cow from a Yisrael, the cow is considered the possession of the real owner with regard to Terumah. Accordingly, if the Kohen is the real owner, the cow may be fed Terumah, and if the Yisrael is the real owner, it may not be fed Terumah. The reason why the rented object is considered the property of the real owner is that the owner suffers the loss in value of the animal in the event that its value decreases or it is harmed.
However, if the one who rents the cow appraises its value with intent to assume full responsibility for it and to return to its owner the exact value after a certain period of time, it is considered the property of the one who rents it, for he bears the monetary responsibility. In such a case, if the person who rents it is a Kohen, he may feed it Terumah.
RASHI explains that this case refers to a rental, like the first case. It differs from a normal case of a rental because at the time of the rental, the person who rents the cow stipulated that he will return the exact value that the cow presently represents (and he will compensate for any difference).
Rashi's explanation is problematic. When the renter accepts upon himself to return the exact value of the object he received, in essence he treats the entire transaction like a loan. However, if he is also paying a rental fee in addition to taking responsibility to return the exact value of the object, the fee that he pays should constitute Ribis, forbidden interest, because he is returning more than he borrowed.
The Gemara in Bava Metzia (69b) explains that in an ordinary case of a rental, the money paid as the rental fee for the object is not considered interest even though the object itself is returned to the owner in addition to the rental fee that he pays. The reason why a rental fee does not constitute Ribis is as follows. Since the renter takes no responsibility for fluctuations in the value of the object, and he takes no responsibility for the loss of the object due to circumstances beyond his control, the owner of the object takes the risk of losing his object. In return for this risk he receives a rental fee, and thus it is not considered interest. Another reason for why a rental fee is not considered interest applies in a case of an object which depreciates in value as it is used. Even if the rental fee cannot be considered compensation for the risk which the owner takes -- such as when the renter accepts upon himself to compensate for the fluctuation in the object's value and for loss due to uncontrollable circumstances (and thus the owner risks nothing when he rents out his object), the rental fee is not considered Ribis because it is considered a form of compensation for the decrease in value of the object as a result of the renter's usage of it.
However, in the case of the Gemara here, the renter takes full responsibility for the value of the cow. Moreover, a cow's value usually remains the same; it does not decrease in value as a result of pulling a plowshare. Why, then, is the rental fee for a cow not considered Ribis? (This is the question of TOSFOS (DH Aval), who rejects Rashi's explanation and says instead that in the second case of the Beraisa, the person borrows the cow and pays no rental fee.)
ANSWER: According to RASHI, perhaps the rental fee for a cow is not considered Ribis for the following reason. A person who rents an animal is not liable if the animal depreciates or dies as a result of normal usage ("Mesah Machmas Melachah"). In the case of the Gemara here, the renter accepted upon himself responsibility to pay only for fluctuations in value that are not work-related. (See also ARUCH LA'NER.)
Perhaps TOSFOS does not accept this explanation because he maintains that if the renter is not responsible to replace the value of the animal under any circumstance, the animal is not considered to be his property with regard to feeding it Terumah. If he is not responsible to replace the animal if it dies as a result of normal usage, it is not considered his property.
Rashi argues that as long as the renter accepts responsibility for secondary liability (other than the actual depreciation due to normal usage), he is considered the owner. Even though he is not considered the owner with regard to responsibility for depreciation as a result of normal wear and tear, he may feed it Terumah. It is for that depreciation in value which he pays a rental fee, and that is why the rental fee is not Ribis. (In the case of an ordinary rental in which the renter does not accept responsibility for any form of decrease in value, the renter is not considered the owner because his rental fee does not suffice to cover the external, secondary factors which might cause it to depreciate.)