As for the evidence cited by the commentator for it, (1) I mentioned this hadith for a useful point: it shows that it is permissible to sell a debt to the person who owes it, subject to two conditions:

The first condition is that it be at its rate on that day.

The second condition is that the exchange be completed before the two parties part. This applies, however, to transactions in which immediate exchange is required, such as dirhams for dinars, dinars for dirhams, wheat for barley, barley for wheat, and the like.

The requirement of immediate exchange in the case of dirhams and dinars, and similar items, is clear. It is clear because, in such a case, one item may not be exchanged for another except through immediate mutual possession, due to the Prophet’s statement concerning gold, silver, wheat, dates, barley, and salt: “When these kinds differ, then sell as you wish, provided it is hand to hand.” (2) As for requiring it to be at its rate on that day, this is so that one does not profit from something for which one has not borne liability, for the Prophet “forbade profit from that for which one has not borne liability.” (3)

An example is where a man owes me dinars, and I sell those dinars to him for dirhams. In exchanging dinars for dirhams, possession is required, though equality in amount is not required, due to the hadith: “When these kinds differ, then sell as you wish, provided it is hand to hand.” He therefore gives me only the dirhams; he is not required to produce the dinars, because the dinars are already owed by him and have thus been deemed received.

An example of its being at its rate on that day is this: if we assume that ten dinars have a market value of one hundred dirhams, then I sell them to him for one hundred dirhams, neither more nor less;