The upshot is that if one of the two currencies is excepted from the other, the sale is invalid according to the school. This is qualified by the case in which the value is liable to rise or fall. But if the value is fixed—such that every ten dirhams equal one dinar—the exception is valid.
“Or if he sells something known together with something unknown whose nature cannot be ascertained, and does not state that each is for such-and-such an amount, the sale is invalid. But if ascertaining it is not impossible, it is valid with respect to the known item in its proportionate share …”
His statement, “Or if he sells something known together with something unknown whose nature cannot be ascertained, and does not state that each is for such-and-such an amount, the sale is invalid”: if he says that each is for such-and-such an amount, it is valid.
For example, he says: “I sell you this she-camel and what is in the womb of another she-camel for one thousand dirhams.” The price is now known, while part of the item sold is known and part is unknown, whose nature cannot presently be ascertained because it is a fetus.
Thus, we say: if he states that each is for such-and-such an amount, it is valid. For instance, if he says: “I sell you this she-camel and what is in the womb of another she-camel for one thousand dirhams: this she-camel is for eight hundred, and the fetus for two hundred,” the sale is valid, because there is no uncertainty in this. But selling the she-camel and what is in the womb of the other for one thousand does not allow us to apportion the price between them, because the value of the fetus in the other she-camel’s womb is unknown and cannot be determined. The price therefore remains unknown, and this is clear.
But if he says: “I sell you this she-camel and her fetus for one hundred: she is for eighty and the fetus for twenty,” they say that this is invalid, because a fetus may only be sold as a subsidiary part of the sale. Once you say, “This she-camel is for eighty and the fetus for twenty,” it has become independent, so its sale is invalid—just as the sale of the fetus in the other she-camel was invalid in the first example—while the mother remains at eighty.