He was asked:

About a man who paid fifty dirhams in advance for one ritl (a unit of weight) of silk, to be delivered at a specified date. When that date arrived, the silk was unavailable. May he accept the value of the silk, or accept anything else in its place?

He replied:

Praise be to Allah. There are two positions reported from Imam Ahmad on this question.

The first is that it is not permissible to accept something else in place of the goods owed under a salam contract (an advance-payment sale). This is also the view of al-Shāfiʿī and Abū Hanīfah, based on the report that the Prophet, may Allah bless him and grant him peace, said:

“Whoever pays in advance for something must not divert it to something else.”

This is the position commonly recognized among the later scholars of Imam Ahmad’s school, and it is the one mentioned by al-Khiraqī and others.

The second view is that this is permissible, just as it is permissible with other debts and with sales of individually specified goods. This is Mālik’s position. Ahmad explicitly stated this in several places, treating goods owed under a salam contract like other goods sold.

Thus, if he accepts a substitute that is sold neither by volume nor by weight, equal in value to the goods owed under the salam contract at the time of substitution, without any excess, or accepts something of the same category in an equal quantity—for example, if he pays in advance for wheat and receives