What is the difference between the following two forms of sale: "I sell you this item for fifty pounds if you pay now, and for sixty pounds if you pay next month," and "This item is for fifty pounds if you pay now, and for sixty pounds if you pay next month," and which of them is considered usury (riba) and thus forbidden, and what is the reason for its prohibition, if any?

The first form of sale mentioned — if it means that the seller sells a commodity for fifty pounds cash, then lends the buyer these fifty to return them as sixty pounds next month — is impermissible, due to its inclusion of explicit usury (riba). This is because returning a loan with an increase is forbidden by consensus, and also because of the condition of the loan contract within the sale contract, which is prohibited and invalidates the sale. As for the second form — which is not agreeing on the payment method at the time of the contract, meaning selling at two different prices for cash and installments without finalization — it is impermissible and falls under the category of "two sales in one sale," which is forbidden by Islamic law, because the price becomes unknown, and an unknown price invalidates the sale. It is a condition in installment sales that the installments be known, not increasing with delayed payment, and that the term be specified.