The explicit wording of the author indicates that we do not revert to an equivalent item, but rather to its value. This is because the seller has acknowledged that it left his ownership in exchange for its value; thus, it is guaranteed by its value. It is not like liability for something destroyed, where one reverts first to an equivalent item and then to its value.
His statement, “the value of an equivalent item”: is this assessed at the time of the contract, the time it perished, or the time of their reciprocal oaths? This depends on whether rescission nullifies the contract from its inception or only from the time it occurs. If we say that it nullifies the contract from its inception, then it is assessed at the time of the contract. If we say that it nullifies the contract from the time of rescission, then the valuation is at the time of rescission. Yet another consideration may conflict with this here: namely, that the item has perished. At the time of rescission it no longer exists, so how can it be valued when it has perished? Accordingly, this would shift to its value at the time it perished. For a person may buy it and not rescind until a week later, by which time the price may have changed. The apparent view is that we revert to its value at the time of the contract, because it is known that during the option period the item sold belongs to the buyer, and any increase in its value belongs to him. The seller transferred it out of his ownership at the time of the contract, so he is entitled only to its value at that time.
What remains to be considered is their disagreement over its value. The apparent wording of the author is that one reverts to the value of an equivalent item, even if it is less than what the buyer stated or more than what the seller stated.
For example, the seller says that it was for one hundred, while the buyer says it was for eighty. The commodity has perished, and we say that they revert to the value of an equivalent item. They then determine that the value of an equivalent sheep is one hundred and fifty, so its value is greater than what the seller claimed; or they determine that its value is sixty, so it is less than what the buyer claimed,