What is the ruling of Islamic law on increasing the price of a commodity for a buyer purchasing on credit without agreeing on the repayment period, since selling it on credit differs from selling it for cash? Is this considered usury (riba)? And how can one atone for past dealings?

Firstly: It is permissible to increase the price for deferred payment over the immediate payment price, and this is not forbidden.

Secondly: Increasing the price in the absence of the customer beyond the price agreed upon at the time of the contract is forbidden and is considered consuming people's wealth unjustly, even if the creditors agree to the increase in exchange for the delay, because it is usury (riba). The difference is between an increase in price at the time of the contract (which is permissible) and a sudden increase in exchange for delaying payment (which is forbidden usury).

Thirdly: The term (period of deferment) must be known if stipulated in the sales contract, to prevent disputes; therefore, selling for an unknown term is not permissible.

Fourthly: Sharia regulations must be observed for the validity of future sales, such as specifying a known term for debt repayment and not increasing the price beyond what was agreed upon in the contract. If the debtor becomes insolvent, they must be given respite until ease, without being obliged to pay an increase.

Fifthly: What pertains to expiation for the past requires knowing the exact reality of what happened, and it is advisable to consult one of the scholars directly. In general, any increase that occurred merely due to delayed payment must be returned to its owner.