Some exchanges require both counter-values to be delivered immediately. In those cases Salam cannot be used to postpone one side.
The example discussed is gold exchanged for silver, where simultaneous delivery is required. Likewise, the form of wheat-for-barley exchange discussed cannot be converted into Salam where immediate mutual delivery is legally required.
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Advance-Purchase Sale (سَلَم): Why Future Delivery Is Allowed
Some exchanges require both counter-values to be delivered immediately. In those cases Salam cannot be used to postpone one side.
The example discussed is gold exchanged for silver, where simultaneous delivery is required. Likewise, the form of wheat-for-barley exchange discussed cannot be converted into Salam where immediate mutual delivery is legally required.