Silver for gold is Riba (usury) unless exchanged on the spot; wheat for wheat is Riba unless exchanged on the spot; barley for barley is Riba unless exchanged on the spot; and dates for dates is Riba unless exchanged on the spot

Mālik ibn Aws, the Tābi‘i, informs that he had gold dinars that he wanted to exchange with silver dirhams. Talhah ibn ‘Ubaydullāh (may Allah be pleased with him) said to him: Give us your dinars to see them. Then, after deciding to buy them, he said to him: Come later when our servant comes to give you the silver dirhams.

This situation took place in the presence of ‘Umar ibn al-Khattāb (may Allah be pleased with him), who expressed his disapproval of such a transaction and swore that Talhah should either pay the silver now or give him back his gold that he took from him.

He justified this by saying that the Messenger of Allah (may Allah's peace and blessings be upon him) mentioned that when selling silver for gold or vice versa, the exchange should be made immediately; otherwise, this will be a prohibited Riba-based transaction and an invalid sale. Thus, gold should not be sold for silver, and silver should not be sold for gold except hand to hand and with the exchange made on the spot.

Moreover, wheat should not be sold for wheat, barley should not be sold for barley, and dates should not be sold for dates except like for like, weight for weight, measure for measure, hand to hand, and nothing thereof should be sold immediately for deferred payment, and it is impermissible for the two parties to depart before the exchange is made.