But if ascertaining it is not impossible—which is the second category—the sale is valid with respect to the known item in its proportionate share, and we deduct the value of the unknown item from the price.
This is called splitting the transaction. It is one of the three types of transaction in which the sale is valid regarding that to which a sale contract can validly apply, but invalid regarding the remainder. Thus, the case under discussion—where ascertaining it is impossible and the sale is valid with respect to the known item in its proportionate share—is one of the cases of splitting the transaction.
A transaction means a contract. This is because, especially in earlier times, when the contracting parties concluded a sale, one would say to the other, “For how much did you sell it to me?” He would reply, “I sold it to you for such-and-such,” and they would clap hands. Thus, a transaction is the contract itself, and splitting it means validating part of it and invalidating the rest. In the case presently before us, where one sells a known item together with an unknown item whose nature is not impossible to ascertain, we have split the transaction: we validate it in what can validly be sold and invalidate it in what cannot.
“If he sells an undivided jointly owned share between himself and another, such as a slave, or something whose price can be apportioned according to its parts, the sale is valid regarding his own share in its proportionate amount. And if he sells his own slave and another person’s slave without that person’s permission, or a slave and a free person, or vinegar and wine, in a single transaction, it is valid regarding his own slave and regarding the vinegar in its proportionate share. The buyer has the option to rescind if he was unaware of the situation.”
His statement, “If he sells an undivided jointly owned share between himself and another,” refers to the second case. “Undivided jointly owned” means jointly owned by him and another person.
His statement, “such as a slave,” also includes a car, land, or anything else jointly and undividedly owned which he sells in its entirety.
For example, this car belongs jointly to my brother and me, and I sell the entire car to someone. I have now sold something jointly owned between myself and another. My sale of my own property is valid, because it is the sale of an owner; but my sale of my brother’s property is invalid, because I am not his agent. Thus, we have split the transaction. We say to the buyer: you now own half the car, because the sale concerning that half is valid. As for the sale of my brother’s share, it is invalid.