His statement, “Entering into a manumission contract with him is not sufficient,” means that if the disbeliever whose slave has embraced Islam says, “I will enter into a manumission contract with him”—and a manumission contract is for the master to sell the slave to himself, as Allah says: “And those of your slaves who seek a contract for emancipation, make such a contract with them if you know there is good in them” [al-Nur 24:33]—then this disbelieving man is saying: “I have no objection to relinquishing ownership of my slave, but I wish to do so through a manumission contract; I will sell him to himself.” We say: This is not sufficient, because a manumission contract does not remove the master’s ownership of the slave until he has paid in full. Before completing payment, he remains the master’s slave. For this reason, a manumission contract is not sufficient.
If he says, “I will sell him, but I wish to stipulate for myself an option to rescind for one month,” this too is not sufficient, because his attachment to him has not been severed: he may say, “I have rescinded the sale.” Thus, if he enters into a manumission contract with him or sells him subject to an option, that does not suffice.
His statement, “If he combines a sale with a manumission contract, or a sale with a currency exchange, it is valid in matters other than the manumission contract,” concerns combining two contracts. If the two contracts are combined as a condition, the contract is invalid; but if they are combined without a condition, the contract is valid.
An example is where he says: “I have sold you this house of mine for one hundred thousand, on condition that you lease me your house for ten thousand.” The other says, “I accept; I have no objection.” The contract is invalid—neither the sale nor the lease is valid—because it makes one contract conditional upon another, and this is invalid. The Prophet ﷺ said: “A loan combined with a sale is not lawful, nor are two conditions in a sale.” (1) This is the position of the madhhab.