Papinianus, Questions, Book X. Paulus states in a note that an exception on the ground of legal ownership may be pleaded in bar of the Publician Action.

Papinianus, Questions, Book XII. Where, after the contract is made, the parties deduct something from the property purchased, this is considered to be included in the original contract, but where they make additions, we do not think that these form part of the contract. This takes place where something is added which supports the purchase; for instance where a bond for double the amount is furnished, or where a bond is furnished together with a surety. But in case the purchaser brings an action where the agreement is not valid, and the vendor also brings one, he will also have the right to avail himself of an exception. The question has very reasonably been asked whether the same rule applies where the price has been subsequently increased, or diminished; since the substance of the purchase consists of the price. Paulus states in a note that where everything remains in its original condition, and an agreement is afterwards made with reference to the increase or diminution of the price, the parties are held to have withdrawn from their former contract, and a new purchase to have been made. 1Papinianus says that where a sale is made in the following terms, namely: “This sale shall be void if it has reference to anything sacred, religious, or public,” and the property is not in public use, but belongs to the Treasury, its sale will be valid, and the vendor cannot avail himself of an exception because it will not be operative.