but difficult to catch, its sale is invalid, because the fish may burrow into the mud and become inaccessible.
His statement, “nor property usurped from anyone other than the usurper or one able to retrieve it”: usurped property is that which has been taken forcibly from its owner. That is, it is not valid for the owner to sell usurped property. Thus, if the owner of the usurped property sells it to a third party, the sale is invalid, because he is unable to deliver it. The author makes an exception, however, when he says, “from anyone other than the usurper or one able to retrieve it.” If it is sold to the usurper—for example, if the owner says to the usurper, “Buy from me what you usurped from me,” and he purchases it—then this is valid, because the effective cause, namely the ability to deliver it, is present: the usurped property is in his possession, so the sale is valid.
This is conditional, however, upon the usurper not withholding it unless the owner sells it to him. If the usurper refuses to return it except through a sale, then the sale is invalid, because it is not entered into willingly, and mutual consent is a condition of sale. That is, if the usurper says, “I will not return it to you; I want you to sell it to me,” and the owner sells it to him under compulsion, reasoning, “I will take the compensation, so that neither my property nor its value is lost,” then the sale is invalid.
If the usurper offers a price many times greater than its value, and the owner sells it to him, is the sale valid or not?
The answer is that it is invalid so long as the owner has not consented, even if he is offered many times its value. The owner may not be willing to sell it to the usurper even if he offers many multiples of its value, because he wishes to see him punished. He knows that, with that price, he could buy ten of the same kind as what was taken from him, yet he wishes to thwart the usurper’s greed and avarice. So he says, “I will never sell it.” In this case, we say that the sale is invalid, even if the price is many times greater.