For example, a person owes another ten dinars and pays him off in dirhams little by little. If, with each dirham he gives him, he reckons it against the dinars, this is valid. Thus, if the ten dinars are equivalent to one hundred dirhams, each time the debtor brings ten dirhams, he says, “These are in exchange for one dinar.” This is valid, because the owner of the dinars has, in fact, taken possession of them, since they are with him. But if he does not say this, and instead brings ten dirhams every day until he has completed and handed over one hundred, then it is not valid.
He said: “If he does not do this, and they subsequently settle their accounts and exchange it at the time of settlement, it is not permissible, because it is a sale of debt for debt. But if one of them takes possession from the other of what is owed to him, then exchanges it for something present and a debt, it is valid” (1).
“If he does not do this” means: if, with each dirham he gives, he does not say, “This is in exchange for such-and-such.” “Then they subsequently settle their accounts” means that, after the dirhams have been fully paid, they settle their accounts. If they exchange it at the time of settlement, it is not permissible, because it is a sale of debt for debt. He has been bringing ten dirhams every day until the hundred is complete, and then says, “Now let us exchange: the one hundred dirhams you have is in exchange for the dinars I owe.” Yet neither dinars nor dirhams are present. It is therefore a sale of debt for debt: the dinars owed by the debtor are sold in exchange for the dirhams received by the creditor. Thus, it is not valid.
The proper way is for one of the two currencies to be present. This is why he said: “But if one of them takes possession from the other of what is owed to him, then exchanges it for something present and a debt, it is valid.” Thus, one may say: “Bring ten dinars,” and then say, “These dinars are in exchange for the one hundred dirhams you have.” The exchange is then between something present and a debt. And if he brings