What is the difference between an agreement to purchase (muwa'adah) and a promise to purchase (wa'd) in Murabahah for the one who commands the purchase, and how can the option [to proceed or not] in the agreement be reconciled with the binding nature of the promise?

The difference between a "promise" (wa'd) and a "mutual promise" (muwa'adah) in Murabaha for the one who commands the purchase is that a promise is issued by one party, whereas a mutual promise is issued by both parties together. A mutual promise is permissible in a Murabaha sale on the condition that at least one of the mutually promising parties has the option to withdraw; otherwise, it is not permissible. This is because a mutual promise that is binding on both parties resembles the sale itself, and it is not permissible for a person to sell what he does not possess. Judicial enforcement of a promise means that the promisor either fulfills the promise or pays compensation for the actual damage resulting from non-fulfillment without a valid excuse.