Allah Most High says in His Book:

“O you who believe, do not consume one another’s wealth unjustly, but only through trade by mutual consent.”

He also says:

“Do not consume one another’s wealth unjustly, nor offer it to the judges so that you may knowingly consume a portion of other people’s wealth sinfully.”

And, in His condemnation of the Children of Israel, He says:

“For their breaking of their covenant”—up to His words—“and their taking of usury, though they had been forbidden it, and their consuming people’s wealth unjustly.”

Whoever consumes people’s wealth unjustly takes one of the two things exchanged without delivering the other. The purpose of financial covenants and contracts is for both parties to take possession: each contracting party asks the other to deliver what was agreed upon. Hence Allah Most High says:

“And fear Allah, in whose name you appeal to one another.”

That is, you enter into covenants and contracts with one another. This is what contracts require and entail, because each contracting party, through the contract, undertakes to provide what the other has requested of him. Contracts therefore impose obligations to deliver possession, and taking possession is what is requested, intended, and sought.

Contracts are thus completed when both parties take possession. Indeed, if two unbelievers embrace Islam, or submit their dispute to our judgment, after both have taken possession under contracts they regard as valid, we do not interfere with those transactions, since the contracts have been concluded through the fulfilment of their requirements.

For this reason, exchanging one deferred obligation for another is forbidden: it creates a contract and imposes obligations without securing the intended result for either party individually or for both together. This is also why Allah has forbidden gambling, which includes sales involving uncertainty. Such uncertainty includes things of which one may be able to take possession